[pyrech]

Same move as jolicode/castor#918. Zensical is the successor of Material for MkDocs, built by the same team.

• mkdocs.yaml converted to zensical.toml.
• The documentation is now built in Docker (docs/Dockerfile) instead of a local Python with poetry: castor doc:build, castor doc:server, and castor doc:build-github-pages run Zensical and mike in the container. pyproject.toml and poetry.lock are gone, as well as the Python setup in the workflow.
• Versioning keeps using mike, through the Zensical-compatible fork maintained by the Zensical team until native versioning ships. The tagged versions keep their mkdocs.yaml, which Zensical reads as-is.
• Stylesheet switched to Zensical/extra.css from jolicode/oss-theme, which keeps the current colors with the modern variant.
• _nav.md, the overrides/ templates and the Dockerfile are excluded from the published site (_nav/ and overrides/*.html were published so far).

Checked locally: the latest build has the same structure and the same 113 code blocks as production, and a full doc:build-github-pages run in a scratch clone produced the same 12 versions and the same versions.json as production.

Two extra commits:

• The GitHub Actions are pinned to the commit of their latest release, and a Dependabot config keeps them updated (monthly, grouped).
• A doc job in the CI builds the documentation on each PR, in strict mode so that a broken link fails the build.

[joelwurtz]

Thanks