docs: publish architecture documentation via mdBook and GitHub Pages (closes #28)

[Dani-giron]

Summary

The Views and Beyond documentation (docs/architecture/: roadmap, quality attributes, directory, module/C&C/deployment views) exists only as plain markdown in the repo, plus a manually maintained copy on the university MediaWiki (valhalla.unizar.es). The wiki copy has already drifted from the source at least once, and nothing renders the mermaid diagrams or cross-links between documents as a browsable site.

Changes

• docs/book.toml: mdBook config, src = "architecture" (no files moved), title/authors, git-repository-url and edit-url-template so each page links back to its source file.
• docs/architecture/SUMMARY.md: navigation, listing the existing README, the three cross-cutting documents, and the three views. No content rewritten to fit it.
• docs/mermaid.min.js and docs/mermaid-init.js: committed rather than generated in CI, since the deploy workflow only runs mdbook build, not mdbook-mermaid install. Without them the three diagrams would silently fail to render on the published site.
• .github/workflows/pages.yml: builds and deploys on push to apotheosis2 (not main) when docs/** changes, or via workflow_dispatch. Downloads pinned prebuilt mdbook/mdbook-mermaid binaries instead of cargo install, to avoid pulling a full Rust toolchain just to build docs.
• .gitignore: added docs/book/, the generated output, never committed.
• Image asset cleanup: one filename had spaces/accents, two PNGs were unreferenced duplicates, two .gitkeep were redundant. Renamed, deduplicated, updated the three affected references in vista-cc.md/vista-modulos.md.
• Several docs described behavior that had since changed in code: hnsw/radix/records documented as pub (now private, [Enhancement] Make Apotheosis internal fields private to protect the synchrony invariant #14), HEURISTIC documented as unvalidated in dump/load (now validated, [Bug] dump() does not encode HEURISTIC in file header #10), empty-index search() documented as panicking (now returns vec![], [Bug] search() panics on empty index #8), and the radix fast-path documented as ignoring k and returning unsorted results (now sorts and truncates to k like the ANN path, [Bug] search() ignores parameter k on radix fast-path #9). Corrected all four.
• Removed leftover internal review notes in HTML comments in two documents, and replaced most exact source line-number references with function/struct names so the docs do not go stale on the next refactor.

Test plan

Verified locally: mdbook build docs succeeds with no warnings, all internal links resolve, mermaid diagrams render, and the site was reviewed end to end with mdbook serve.

After merging, two one-time repository settings are needed for the site to go live: enable Pages with source set to GitHub Actions, and allow the apotheosis2 branch to deploy under the github-pages environment's protection rules.

What to do with the existing valhalla.unizar.es mirror is intentionally out of scope here; that involves university infrastructure outside this repo and will be proposed separately once Pages is confirmed working.

Closes #28.