ci: build docs from SUMMARY.md rather than by walking the directory

[AlexMikhalev]

What

deploy-docs.yml invokes md-book as md-book -i . -o book. That points at the docs root, which has no SUMMARY.md, so md-book falls back to walking the directory: 574 pages in path order, publishing archive/, artifacts/ and research notes alongside the real documentation.

md-book build . reads book.toml, so it honours src = "src" and the 118-line SUMMARY.md: 59 pages, in the declared order, with the part titles and the [output.html.fold] the config already asks for. Output still lands in docs/book/, which is what the upload step expects.

Also drops the || true, which was hiding build failures behind a green job and a stale artefact.

Why now

terraphim/md-book#27 just landed, adding the SUMMARY.md book model and the build [dir] subcommand. This workflow clones md-book from main, so it picks that up automatically.

That PR also fixes two things visible on docs.terraphim.ai today:

• git-repository-url and edit-url-template are set in docs/book.toml but md-book knew only its own key names, so the repository and edit links have been silently missing. Both mdBook spellings are now accepted.
• A SUMMARY.md entry pointing at ../CLOUDFLARE_DEPLOYMENT.md (a file beside book.toml) used to abort the summary-driven build; containment is now scoped to the book directory rather than src/.

Verification

Built this repo's docs with the merged md-book:

md-book build .   ->  Total pages: 59, into docs/book/

Sidebar carries the SUMMARY part titles (Crates Overview, Components, Integrations); orphaned files under docs/src are reported rather than silently published.

Note mathjax-support = true in docs/book.toml warns as unimplemented — no content in docs/src uses maths, so nothing is lost; remove the key or leave it as a marker.