docs!: build and version the online documentation with Zensical

[nandgator]

What

Moves the online documentation from MkDocs (+ Material) to Zensical, keeping the same content, navigation and mike-style versioning on the gh-pages branch.

• docs-online group is now zensical, mkdocstrings-python and mike.
• mike comes from Zensical's compatible fork ([tool.uv.sources], pinned by uv.lock). Zensical has no native versioning yet; this is the documented bridge and writes the same gh-pages/versions.json layout, so existing versions are kept.
• mkdocs.yaml → mkdocs.yml (the name Zensical and mike look for); the existing config is otherwise unchanged. package/export doc runs zensical build.
• Dropped: mkdocs, mkdocs-material, mkdocs-git-revision-date-localized-plugin (and its gitpython etc.). The "last updated" date on pages goes away; Zensical has no equivalent plugin yet.
• Man page, Sphinx docs and the package build are untouched.

Testing

• uv lock regenerated; uv run --locked --group docs-online python package/export doc builds with "No issues found" and leaves tracked files unchanged.
• API pages compared against the MkDocs build earlier: same headings and code blocks (about 4% size difference).
• Versioned mike deploy/serve flow tried locally with the fork.

Risks

• Zensical's mkdocstrings support is labelled preliminary (no backlinks yet).
• The mike fork is a git dependency, described upstream as temporary.
• Merging this does not deploy anything. On the fork, the docs workflow deploys dev on the next push to master, which is a good first check of the result before the 0.36 release deploy.

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