[FreeApophis]

Resolves #1046

main already contains the 4.0 breaking changes and every push to main deploys the book, so the live documentation currently describes unreleased behaviour (and λ1202 / λ1203, still reported by the shipped analyzers, return 404).

What changes

• Two books in one deployment. GitHub Pages is deployed as a single artifact, so one workflow run builds every version. The workflow checks out the 3.x documentation (STABLE_DOCUMENTATION_REF, the branch release/3.8.x) next to main.
  • /funcky/book/ is the stable 3.x book (default)
  • /funcky/book/4.x/ is the book from main, labelled "4.x (preview)"
• Version switcher. Website/book/version-switcher.js is injected into every book by the build script, so the 3.x sources need no change. It stays on the same page when the other version has it and falls back to the start of that book otherwise.
• Analyzer rule redirects. The analyzers link to /funcky/analyzer-rules/{id}.html, but only λ0001 had a redirect. The build now generates one for every rule page found in any book; a rule goes to the default book, or else to the first book that documents it.
• Folding (Fold the subchapters in the rendered documentation #1047) is applied to all books through the build script, so that the 3.x book gets it too.
• workflow_dispatch is added to redeploy after a change to the 3.x documentation.

When 4.0.0 is released

Swap the two entries of books in Website/build.fsx: 4.x moves to book, 3.x to book/3.x, 4.x is listed first.

Notes

• 3.x documentation fixes go to release/3.8.x; the companion PR against that branch redeploys the website after such a change.
• The old docs.yml on release/3.8.x would deploy the single-book site on a 3.x tag; the companion PR removes that.
• Locally I verified that the script compiles and generates the 27 redirects; mdBook is not installed locally, so the book builds and the switcher are only exercised by CI.