[docs] Replace `mkdocs` by `properdocs` ?

While testing a PR, I built the a2ui.org site with mkdocs, and got the following warning:

```console
(.venv) dit@dit:/work/google/a2ui$ mkdocs serve

│ ⚠ Warning from the Material for MkDocs team
│
│ MkDocs 2.0, the underlying framework of Material for MkDocs,
│ will introduce backward-incompatible changes, including:
│
│ × All plugins will stop working – the plugin system has been removed
│ × All theme overrides will break – the theming system has been rewritten
│ × No migration path exists – existing projects cannot be upgraded
│ × Closed contribution model – community members can't report bugs
│ × Currently unlicensed – unsuitable for production use
│
│ Our full analysis:
│
│ https://squidfunk.github.io/mkdocs-material/blog/2026/02/18/mkdocs-2.0/

----------------------------------------------------------------

WARNING: MkDocs may break support for all existing plugins and themes soon!

The owner of MkDocs has completely abandoned maintenance of the project, and instead is
planning to publish a "version 2" which will not support any existing themes, plugins or even
your configuration files. This v2 may eventually replace what you download with `pip install
mkdocs`, suddenly breaking the build of your existing site.

To avoid these risks, switch to ProperDocs, a continuation of MkDocs 1.x and a drop-in
replacement that supports your current MkDocs setup.
Simply install it with `pip install properdocs` and build your site with `properdocs build`
instead of the MkDocs equivalents.

Alternatively, to just skip this warning in the future, you can set the environment variable
`DISABLE_MKDOCS_2_WARNING=true`.

For more info visit https://github.com/ProperDocs/properdocs/discussions/33 and
https://properdocs.org/

(This warning was initiated by one of the plugins that you depend on.)
```

I guess we need to migrate to ProperDocs?