[doc] Add legacy-version 404 redirect coverage to current.yaml

[dstrodtman]

Why

A 6-month GA4 audit of docs.ray.io found 2,017 out-of-support legacy-version paths receiving ~552K views with no redirect rule and no equivalent on current docs. Under any legacy-cohort cutover, that traffic lands on 404s. This PR adds the redirect coverage for the bulk of that gap.

What

21 page catch-all rules appended to doc/redirects/current.yaml, covering ~96% of the gap by views and by paths. Each cluster lands on its nearest surviving section index:

• Autodoc / API refs (RLlib package_ref, Serve, Data, Tune, Train, Ray Core, Observability) → that library's API reference index.
• Examples (Ray AIR, Tune, Train, Ray Core) → the examples gallery.
• Restructured sections (Kubernetes, Serve tutorials) → the section index.
• Deprecated, no current equivalent: Ray AIR → AIR getting-started; Ray Workflows → Ray overview.
• Sphinx _modules/ source views → the Ray source on GitHub (a view-source request for a symbol that no longer exists is better served by GitHub search than a 404).
• Three targeted rllib-* overrides precede the RLlib catch-all so high-traffic renamed pages land on their real equivalents.

Design

All rules are type: page with force omitted (defaults to false), so each fires only when the path would 404. Pages that still exist with the same name on any version resolve untouched; only moved/deleted/renamed paths hit the catch-all. This composes with the planned legacy-cohort cutover (/en/releases-X.Y.Z/* → /en/latest/:splat): the cutover preserves same-name paths, and these rules catch what no longer exists on current docs.

Validation

rtd-redirects validate doc/redirects/current.yaml
→ 0 error, 49 warning

Zero ordering/shadow errors. The 49 chain warnings are benign: each is an existing rule whose to is a live page that happens to sit under one of the new section wildcards, which never fires on a live page given force: false.

Notes

• Follows the source-of-truth workflow established in [doc] Establish doc/redirects/current.yaml as the redirects source of truth #63367.
• Application to Read the Docs is a manual maintainer step post-merge per doc/redirects/README.md.

[gemini-code-assist]

Code Review

This pull request updates the documentation and adds 21 legacy-version 404-coverage catch-all redirect rules (DOC-892) to handle high-traffic, out-of-support documentation paths for various components like RLlib, Serve, Data, Ray AIR, Tune, Train, and others. Feedback on the changes suggests updating the /rllib/ redirect target to /rllib/index.html to ensure consistency with other rules and avoid potential double-redirects or canonical URL resolution issues.

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[gemini-code-assist]

For consistency with other redirect rules in this file (which point to specific .html files or index.html rather than trailing slashes), and to avoid potential double-redirects or canonical URL resolution issues, please redirect to /rllib/index.html instead of /rllib/.

  to: /rllib/index.html

[dstrodtman]

Adopted to: /rllib/index.html for consistency with the rest of the file — the existing /rllib.html and /rllib-toc.html rules already target /rllib/index.html. Applied in the latest push.

One note for the record: the double-redirect concern doesn't actually apply here. /en/latest/rllib/ returns 200 in a single hop, and the trailing slash is the server-canonical form (/en/latest/rllib 302s to /en/latest/rllib/, not the reverse). So /rllib/ wouldn't have chained — but index.html is still the right call for consistency.