[doc] Convert observability, more-libs, references, and AIR pages from RST to MyST

[dstrodtman]

Why are these changes needed?

This PR converts the remaining prose pages under ray-observability/, ray-more-libs/, ray-references/, and ray-air/ from reStructuredText to MyST Markdown. It continues the RST-to-MyST conversion that already landed for ray-core/ (#66430), data/ (#66375), and the doctest-wired pages (#66461). The conversion is format only: the goal is for each page's rendered HTML to match /en/master.

There are five commits:

1. Conversion. This commit converts 25 pages, git rms the .rst sources, and adds one doc/BUILD.bazel exclude, which commit 5 reverts. Prose keeps its source line wrapping, so the diff lines up with the .rst line by line.
2. Soft-wrap. This commit reflows whitespace only, to one line per paragraph and list item. verify.py from ray-soft-wrap confirms that the non-whitespace bytes and the rendered HTML don't change on all 25 files.
3. Render fixes. This commit fixes six regressions that the render diff against /en/master caught, even though the build was green. In modin, two bare-word RST link references (Modin_, documentation_) rendered as literal text. In getting-started, a bold URL was autolinked. In system-metrics, a URL in a table cell lost its link. In dask-on-ray, _not_ rendered as italics. In glossary, the inline .. TODO: RLlib text, which /en/master shows, had become a comment. The commit also adds compat anchors to five headings with apostrophes (see below).
4. Pseudo-lists. This commit restores three pseudo-lists that the soft-wrap merged into one line.
5. Review feedback. This commit removes the glossary's placeholder TODO entries, reverts the doc/BUILD.bazel change so the PR stays docs-only, and fixes defects that the review flagged in the .rst sources: a stray definition list in dask-on-ray, an unconverted link in system-metrics, an extra parenthesis in debug-failures, typos in debug-hangs and optimize-performance, a grammar slip in ray-debugging, and em-dash pseudo-list items in general-debugging, which are now a bulleted list.

Scope

• ray-air/: getting-started and deployment, converted together. getting-started includes deployment, and the include now points at deployment.md.
• ray-observability/: 14 pages, including user-guides/debug-apps/ and reference/system-metrics.
• ray-more-libs/: 9 pages.
• ray-references/: glossary and faq.

Left as RST: ray-observability/reference/api.rst (autosummary) and ray-observability/reference/cli.rst (click). They're API reference pages, so they stay RST along with the other */api/ reference trees.

Construct notes for reviewers

• Glossary. The glossary is a native MyST {glossary} directive, not RST wrapped in {eval-rst}. Sphinx's glossary directive parses each definition with the host parser, so the definitions are Markdown now. The glossary's placeholder .. TODO: entries are removed. Missing terms can be identified separately.
• Glossary doctest. Nothing ran the glossary's .. doctest:: block as .rst: pytest-sphinx doesn't execute doctest sections, and --doctest-glob='*.md' doesn't match .rst. As .md, the stock doctest parser would start running its five >>> prompts, and the [[0],[1],...,[3],[4]] output can't pass without ELLIPSIS. Each prompt now carries # doctest: +SKIP, which keeps coverage the same as master without a doc/BUILD.bazel change. Sphinx's default trim_doctest_flags strips the flag from the rendered page, as in data/working-with-tensors.md. If you'd rather fix the expected output and start testing it, I can do that in a follow-up.
• FAQ. faq.md includes _includes/_help-links.md, the Markdown copy of _help.rst that [docs] Convert ray-core scheduling pages to MyST #66417 added. _help.rst stays, because cluster/vms/user-guides/community/yarn.rst and tune/key-concepts.rst still include it.
• Comparison table. The grid table in ray-collective-custom-backend is now a native {list-table}.
• Line blocks. The line blocks in dask-on-ray's version table are now Markdown hard line breaks. This is an expected render difference: <p> with <br> instead of div.line-block.

Dead links fixed on the way

These links were broken on /en/master and fail fail_on_warning when they're transcribed into MyST:

• joblib and multiprocessing linked to relative actors.html and installation.html, which resolve under ray-more-libs/ and return 404. They now link to {doc} /ray-core/actors and /ray-overview/installation.
• ray-collective linked to #api-reference, but the heading's anchor is #api-references.

Open PRs that edit these pages

Each of these PRs needs to move its change onto the .md file: #66363 (ray-event-export), #65693 and #65583 (system-metrics), and #64957 (joblib, multiprocessing). #65321 edits reference/cli.rst, which this PR doesn't convert.

Related issue number

None.

Checks

• I've signed off every commit (git commit -s).
• Duplicate check: no open PR converts these pages. I searched open PRs for MyST, observability, glossary, and more-libs, and for open PRs that edit these files.
• Doctest parity: doctest_parity.py (the pytest-sphinx parser, run against upstream/master) reports the same sections and examples on all 25 pages, before and after the soft-wrap: 12 examples before, 12 after.
• Local doctest: pytest --doctest-modules --doctest-glob='*.md' -c NO_PYTEST_CONFIG -p doctest_pytest_plugin on the 24 converted pages in the main target, with Ray nightly bc900c2 on Python 3.10: 5 passed. Those are the 5 files with examples. The glossary, now in the target too, reports 1 skipped: the stock parser finds 5 examples and all 5 carry +SKIP.
• Static checks: frontmatter parses, fences balance, every label is preserved, and there's no leftover RST outside fences or directives.
• RtD preview: green under fail_on_warning on all three commits.
• Render diff (render_diff.py, preview against /en/master, filtering the two class-only differences every MyST page shows): 11 of 25 pages are identical. Every difference on the other 14 is one of these:
  • RST comments now emit <!-- ... --> HTML comments, which readers don't see. Pages: ray-air/*, ray-more-libs/index, ray-collective, getting-started, general-debugging, glossary.
  • The dask-on-ray version table: Markdown hard line breaks (<p> with <br>) replace RST line blocks (div.line-block).
  • The dask-on-ray "Why use Dask on Ray?" item keeps its definition-list structure. MyST adds a myst class to the <dl>, and the class has no styling.
  • Same-page fragment and {doc} links render as reference internal instead of reference external. Pages: ray-collective, joblib, multiprocessing. This list also includes the dead links fixed above.
  • Headings with an apostrophe get a new anchor, because MyST drops the apostrophe where docutils turned it into a hyphen. For example, #what-s-the-out-of-memory-error becomes #whats-the-out-of-memory-error. Five of these headings had no label, so each gets a compat target with the old slug, which keeps existing links working. The sixth, in optimize-performance, already carries the dashboard-cprofile label.

This PR was written with AI assistance (Claude Code). I reviewed the changes and ran the checks above.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request converts several documentation files from reStructuredText (RST) to MyST Markdown format across Ray's libraries and observability guides. The review feedback highlights several formatting and rendering issues introduced during this migration, such as leftover RST syntax, broken bulleted lists due to incorrect line reflows, links wrapped in backticks rendering as literal code, typos, grammatical errors, and incorrectly converted TODO comments. Addressing these issues will ensure the documentation renders correctly in MyST Markdown.

[gemini-code-assist]

The colon at the start of line 97 is a leftover from the RST :ref: role conversion. It should be removed, and the indentation should be cleaned up to ensure correct rendering in MyST Markdown.

[gemini-code-assist]

The bulleted list items have been reflowed into a single line with escaped hyphens (\-), which breaks the list rendering in Markdown. They should be split back into a proper bulleted list.

[gemini-code-assist]

The link is enclosed in backticks, which will render as literal inline code instead of a clickable hyperlink. It should be converted to a standard Markdown link.

[gemini-code-assist]

There is an extra closing parenthesis ) after the Markdown link [Python pdb](https://docs.python.org/3/library/pdb.html)). It should be removed.

[gemini-code-assist]

Typo in the error message description: "No such file or direction" should be "No such file or directory".

Suggested change

	* If you see "No such file or direction", check if your worker process has exited.
	* If you see "No such file or directory", check if your worker process has exited.

[gemini-code-assist]

Two list items starting with em-dashes (—) have been reflowed into a single line, which prevents them from rendering as a list. They should be split into separate lines and formatted as a standard Markdown bulleted list.

Suggested change

	— Use only shared file paths for such applications. For example, a network file system or S3 storage can provide the required consistency. — Avoid relying on local files to be consistent across machines.
	- Use only shared file paths for such applications. For example, a network file system or S3 storage can provide the required consistency.
	- Avoid relying on local files to be consistent across machines.

[gemini-code-assist]

Two list items starting with em-dashes (—) have been reflowed into a single line, which prevents them from rendering as a list. They should be split into separate lines and formatted as a standard Markdown bulleted list.

[gemini-code-assist]

Typo: "Submit" is capitalized in the middle of the sentence. It should be lowercase "submit".

Suggested change

	If you are still experiencing a slowdown, but none of the above problems apply, we'd really like to know! Create a [GitHub issue](https://github.com/ray-project/ray/issues) and Submit a minimal code example that demonstrates the problem.
	If you are still experiencing a slowdown, but none of the above problems apply, we'd really like to know! Create a [GitHub issue](https://github.com/ray-project/ray/issues) and submit a minimal code example that demonstrates the problem.

[gemini-code-assist]

Grammatical issue: "It allows to set breakpoints" should be "It allows you to set breakpoints" or "It allows setting breakpoints".

Suggested change

	Ray has a built in debugger that allows you to debug your distributed applications. It allows to set breakpoints in your Ray tasks and actors and when hitting the breakpoint you can drop into a PDB session that you can then use to:
	Ray has a built in debugger that allows you to debug your distributed applications. It allows you to set breakpoints in your Ray tasks and actors and when hitting the breakpoint you can drop into a PDB session that you can then use to:

[gemini-code-assist]

The RST comment .. TODO: RLlib was incorrectly converted to .\. TODO: RLlib, which will render as literal text. It should be converted to a standard Markdown comment <!-- TODO: RLlib -->. Note that the same issue applies to other .. TODO: lines in this file (e.g., lines 145, 157, 163, etc.) and they should all be converted to standard HTML comments <!-- TODO: ... -->.

Suggested change

	A full training epoch can consist of multiple training iterations.
	<!-- TODO: RLlib -->