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

[Dani-giron]

Closes #28.

Sets up mdBook over the existing docs/architecture/ Views and Beyond documentation and adds a GitHub Actions workflow that deploys it to GitHub Pages on push to apotheosis2.

No content was moved or restructured to fit mdBook; book.toml points src directly at the existing architecture/ folder, and SUMMARY.md just lists the documents that were already there.

Included in this PR, see #28 for details on each:

• mdBook config, navigation, and the GitHub Pages deploy workflow (pinned prebuilt binaries, no Rust toolchain needed in CI)
• Mermaid runtime assets committed directly (the workflow does not run mdbook-mermaid install)
• Image asset cleanup: one filename with spaces/accents renamed, two unreferenced duplicate PNGs removed, two redundant .gitkeep removed
• Four places where the docs still described pre-fix behavior (pub fields, unvalidated HEURISTIC, empty-index panic, unsorted fast-path) corrected to match current code, referencing issues [Enhancement] Make Apotheosis internal fields private to protect the synchrony invariant #14, [Bug] dump() does not encode HEURISTIC in file header #10, [Bug] search() panics on empty index #8, [Bug] search() ignores parameter k on radix fast-path #9
• Leftover internal review notes removed from two documents; most exact source line-number references replaced with function/struct names

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.