Closes #21635: Migrate from mkdocs to Zensical (#21742)

* 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

docs/_theme/partials/copyright.html

@@ -1,18 +0,0 @@
-<div class="md-copyright">
-  {% if config.copyright %}
-    <div class="md-copyright__highlight">
-      {{ config.copyright }}
-    </div>
-  {% endif %}
-  {% if not config.extra.generator == false %}
-    Made with
-    <a href="https://squidfunk.github.io/mkdocs-material/" target="_blank" rel="noopener">
-      Material for MkDocs
-    </a>
-  {% endif %}
-</div>
-{% if not config.extra.build_public %}
-  <div class="md-copyright">
-    ℹ️ Documentation is being served locally
-  </div>
-{% endif %}