docs: backfill the 2.1.0 docs version and guard release snapshots

[nicklambourne]

Why

The docs site serves the current package version live (lastVersion: "current",
labelled from python/pyproject.toml) and lists every earlier release as a
frozen snapshot in the version dropdown. Cutting those snapshots is a manual
docusaurus docs:version step — and nothing in the release procedure or CI ever
performed it. So when 2.1.0 was released, no snapshot was frozen; 2.1.1
then shipped as the live current version, and 2.1.0 silently disappeared from the
dropdown (it was never in docs/versions.json, and it is newer than the legacy
2.0.0 suffix the legacy porter manages).

Part 1 — Backfill the 2.1.0 snapshot

• Froze 2.1.0 from the current docs tree (its content is unchanged since the
  2.1.0 release: git diff --quiet python/v2.1.0 HEAD -- docs/docs is empty)
  using the standard docusaurus docs:version 2.1.0 flow.
• Generated the Python and TypeScript API reference first (pnpm generate
  plus the typedoc build step), so the snapshot carries a real bilingual 2.1.x
  reference — the generated pages are gitignored under docs/docs/ but are
  committed inside versioned_docs/version-2.1.0/.
• docs/versions.json now lists 2.1.0 ahead of the frozen legacy suffix. This
  is exactly the shape the legacy generator preserves: write_registered_versions
  keeps non-legacy versions at the front (preserved + registered), so the
  snapshot is stable under regeneration.

No legacy version, the legacy manifest, or any existing versioned_docs /
versioned_sidebars entry was touched.

Part 2 — Close the process gap

• RELEASING.md — added a first step to the coordinated release procedure:
  freeze the outgoing docs version with docusaurus docs:version <outgoing>
  before bumping, and explained the lastVersion: current model (the latest
  release is live; each release freezes the previous version).
• docs/scripts/check_release_snapshots.mjs (new) + check:release-snapshots
  script, wired into .github/workflows/docs.yml — enumerates the released
  python/v* tags and fails if any, other than the current package version, is
  missing from docs/versions.json or the legacy manifest. Dependency-light
  Node, matching the neighbouring check:*.mjs scripts; the workflow already
  checks out with fetch-depth: 0 so the tags are present.

With 2.1.0 backfilled the guard passes (2.1.1 is the current live version, so
exempt); it fails loudly for any future release that forgets its snapshot.

Verification

• pnpm --filter @slackblocks/docs build succeeds (Python + TS reference,
  Docusaurus build, TS API rendering, legacy fixture, legacy contract).
• Generation-clean guard is clean: after a fresh generate + build,
  git diff --exit-code -- docs/legacy/manifest.json docs/versioned_docs docs/versioned_sidebars docs/static/img/legacy returns 0.
• check:release-snapshots passes now, and fails with exit 1 (naming 2.1.0)
  when the version is removed from versions.json.
• The whole diff only adds the version-2.1.0 snapshot, its sidebar, the
  versions.json line, and the Part 2 files.

🤖 Generated with Claude Code