Publish versioned release websites and templated resource links

[helly25]

Publish bashtest documentation as immutable static websites for each release, with a root redirect to the latest stable release. Historical releases can be backfilled without making a new release or moving a tag.

AG;DR

• Add release-site.json mapping repository source paths to published HTML/assets under site/tag/<exact-tag>/. Convert tagged Markdown, rewrite configured links, copy assets, and reject missing Markdown mappings or broken internal files/anchors before retaining a snapshot.
• Publish after successful release workflows or manual dispatch. Backfill accepts an explicit tracked configuration from main while all content still comes from the original tag; retain the configuration, checksum, and source commit with each immutable snapshot.
• Preserve the entire retained Pages tree on every deployment. The root follows the latest stable release; older snapshots remain available and retries do not rewrite them.
• Render release-note resource links from .github/release-notes.md.template using tools/release_notes.sh TAG. The website link keeps the exact tag spelling.
• Add 24 site regression tests and two release-note tests (with stable, v-prefixed, prerelease, and invalid-tag cases). Main CI also converts the configured documents and validates generated links in a disposable preview; it does not publish.
• No coverage link is added because this repository currently has no coverage publisher.

Validation: all 26 site/template tests and focused pre-commit checks passed locally. Real Markdown conversion and complete generated-link audits passed for the repository documentation. GitHub CI will validate the pushed head. This PR does not deploy a site or create/change a release tag.

Based on the implementation in mbo #433, independently included here with no merge dependency. The original mbo 0.15.0 tag was successfully backfilled and deployed before merge: live snapshot, deployment verification. Historical tags with different document layouts need a compatible explicit configuration; absent historical sources never fall back to current content.

Coordinated rollout: mbo #433, xff #783, carve #119, coderef #113, bzl #61.