feat: add auto-generated "See also" links to docs pages

[zoltan-baba]

Summary

• Adds an auto-generated "See also" section to every docs page, using local sentence-transformer embeddings (no external API calls) to surface related pages by cosine similarity, capped per source directory so tightly-clustered sections don't crowd out cross-cutting matches. Writers can override or exclude picks per page via see_also/see_also_exclude frontmatter.
• Resolves each related link live, per-locale, from Docusaurus's own doc data — see_also.json stores only a relatedness graph of doc ids, never a baked title/permalink. This is what makes the same data correctly serve both /en/ and /ja/ without regenerating per locale, and it's why a stale or removed reference is silently dropped at render time instead of ever becoming a dead link.
• Adds CI automation across three workflows: a cheap, dependency-free prune of stale ids on any PR that deletes or renames a page (skips cleanly on fork PRs, which can't push back); a cost-gated scheduled full re-embed that only runs once ~40 docs files (excluding generated API reference pages) have changed since the last regeneration — measured against this repo's actual edit history to land around once a week; and a test suite for the prune/pruning logic itself.

Test plan

• npx tsc --noEmit passes — no type errors in the SeeAlso component's Docusaurus hook usage
• python3 scripts/test_prune_see_also.py — 14/14 passing (id derivation, diff parsing, delete/rename/no--M-fallback edge cases)
• Regenerated see_also.json from a clean npx docusaurus build --locale en and confirmed the id-only output shape
• Verified in a local dev/build preview that See also links render with correct titles and resolve to the right locale on both /en/ and /ja/ builds — no doubled locale prefixes
• Verified the CPU-only torch install order in regenerate-see-also.yml actually avoids reinstalling a CUDA build
• All three new/changed workflow YAML files parse correctly
• Not yet exercised inside actual GitHub Actions (only simulated locally) — worth watching the first real run of each workflow after merge

[github-actions]

Preview: https://feature-see-also-embeddings.bitrise-docs.pages.dev

[ilanazholobovsky]

Read all 13 code files; verified on the preview that the section renders correctly on both /en/ and /ja/ (same id-only graph, locale-correct permalinks, no doubled prefixes). The id-only design and the render-time drop hold up exactly as described. Approving — three non-blocking notes, in descending order of importance:

1. prune-see-also.yml diffs against the base tip, not the merge-base. git diff -M "$BASE" HEAD with BASE = github.event.pull_request.base.sha is a two-dot diff against main's current tip, so main-side changes since the branch point leak in reversed: a page added on main shows up as D (usually a no-op here, since the branch's see_also.json predates that id), but a page renamed on main shows up as R and gets remapped in the PR's stale copy — a committed change that then conflicts with main's own see_also.json. Three-dot fixes it: git diff --name-status -M "$BASE...HEAD" -- docs. The test suite can't catch this one — it sits above the parse_changes boundary.
2. The bot's [skip ci] commit becomes the PR head with no check runs. If required checks are ever enforced on this repo, a pruned PR can hang on "Expected" statuses, since the head commit skipped everything. Worth adding to the "watch the first real runs" list from the test plan.
3. see_also / see_also_exclude (and the bare locale-less path form) are only documented in generate_see_also.py's docstring. CLAUDE.md is where authors and agents will actually look — the same gap #203 just closed for diff blocks. Fine as a follow-up ticket rather than in this PR.