Docs: migrate versioned-docs publishing to myst-version-switcher assemble model (v0.5.0)

[coretl]

Migrate this repo's versioned docs publishing onto the new myst-version-switcher-plugin assemble model (v0.5.0), replacing the legacy peaceiris/actions-gh-pages + keep_files gh-pages deployment. This also serves as a real-world test of the plugin's tutorial + migrate-from-gh-pages docs.

In this PR (base: docs branch)

• Add _docs.yml + ci.yml + _publish.yml (the documented tutorial split), adapting the build step to this repo's toolchain.
• Build packs docs.zip (bare html/ root); _publish.yml runs assemble@v0.5.0 → upload-pages-artifact → deploy-pages (no gh-pages branch).
• During migration the docs branch is published as the live version (main has no MyST docs yet): docs is added to the publish triggers and assemble runs with guard-default-branch: false. Cross-repo objects.inv refs stay on /docs.
• Add the version-switcher plugin v0.5.0 release URL to docs/myst.yml; add docs/navbar_end.md with the version-switcher directive; retire .github/pages/make_switcher.py. Build keeps npm i -g mystmd@1.10.1 + myst build --strict.

Deferred to a later cutover (NOT this PR — needs repo admin, changes live docs)

• Run scripts/migrate.sh <org/repo> to backfill docs.zip onto existing release tags, flip Settings → Pages → Source to GitHub Actions, set the github-pages environment's deployment branches to No restriction, verify every switcher.json URL returns 200, then delete gh-pages.

PR to follow, targeting the docs branch.

[coretl]

Branch docs-version-switcher-assemble ready (PR to follow into docs).

What changed:

• ci.yml + _docs.yml + _publish.yml (tutorial split); build driven through make docs, packs docs.zip (bare html/ root); _publish.yml runs assemble@v0.5.0 → deploy.
• Added a minimal Makefile + CONFIG.example (the repo had none) exposing the same make docs target as the other repos; gitignored the local CONFIG.
• .devcontainer/Dockerfile (kas-based, unchanged base): added make + npm so make docs works in the container.
• docs/myst.yml: added the version-switcher plugin v0.5.0; added navbar_end.md. Retired .github/pages/make_switcher.py.
• Smoke-built locally: docs build clean, switcher widget renders.
• docs published as the live version during migration (guard-default-branch: false).

Note: this repo has only prerelease tags, so there is no non-prerelease "preferred"/stable target yet — the switcher will list docs (+ any backfilled versions) until a release is cut.

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Cutover sequence (full migration, this round)

Once the PR is merged to docs (so the new workflow is live), per the version-switcher migrate-from-gh-pages how-to:

1. scripts/migrate.sh PandABlocks/<repo> --dry-run — review the backfill plan.
2. scripts/migrate.sh PandABlocks/<repo> — backfill docs.zip onto each release tag from its gh-pages dir, flip Settings → Pages → Source to GitHub Actions, set the github-pages environment deploy branches to No restriction.
3. Re-run the docs-branch CI so its publish job deploys (it injects the docs/ build + gathers the backfilled releases). Note: the assemble model gathers default branch + releases + open PRs; the docs version enters only via this in-run injection, so the deploy must come from a docs push/re-run, not a bare _publish dispatch.
4. Verify every switcher.json URL returns 200.
5. Delete gh-pages (gated behind the verification + explicit confirmation).

Rollback between steps 2 and 5: flip Pages source back to gh-pages.

[coretl]

✅ Cutover complete. Pages source flipped to GitHub Actions; site reconstructed by assemble@v0.5.0 and deployed.

Live now (all 200): main (legacy, via shim), docs (new MyST), and backfilled prereleases 4.2b1 + 4.1alpha1. All tags are prereleases, so preferred/★ + root redirect = main for now.

Notes:

• The ~13 stale intermediate 4.1aN/4.1alphaN gh-pages dirs (old CI builds, not GitHub Releases) are dropped from serving — they remain on gh-pages and are recoverable. Switcher trimmed from 16 → 4 entries.
• Added a minimal Makefile + CONFIG.example exposing make docs; kas devcontainer + make/npm.
• Closed Docs #11 (docs→main) to stop its preview racing the docs deploy.

Deferred (until docs is ready to merge into main)

• Remove the Stage legacy main/ from gh-pages shim step in _publish.yml.
• Delete the gh-pages branch (retained for now as the /main/ content source and the instant rollback — flip Pages source back to "Deploy from a branch" to roll back losslessly).
• Re-open / recreate the closed docs→main PR when actually merging.