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

[ditman]

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

(.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?

[ditman]

/cc @zeroasterisk have you seen this before?