* Drop mkdocs from `requirements.txt` and add Zensical
* Replace mkdocs with Zensical in CI and pre-commit tasks
* Remove `.info` from the `docs/` build directory (obsolete)
* Update the legacy ReadTheDocs configuration
* Update upgrade script to use Zensical
* Remove custom docs footer
* Remove obsolete CSS
Pin Ruff to v0.15.2 in CI and pre-commit to avoid breakages from
upstream releases. Run Ruff via astral-sh/ruff-action (pinned by SHA)
instead of installing Ruff via pip.
Document where Ruff is pinned and keep the release checklist/style guide
in sync.
Fixes#21472Fixes#21497
