Investigate properdocs replacement-warning nag pulled in by mkdocs plugins #94

What happened

During pre-0.4.0 release regression testing, mkdocs build --strict (and mkdocs serve) printed this to stderr on every run:

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. ...

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

What I found

• A separate PyPI package properdocs (found 1.6.7 in the resolved environment) gets installed transitively and lists itself as Required-by for several of our real dependencies: mkdocs-gen-files, mkdocs-literate-nav, mkdocs-redirects, mkdocs-rss-plugin, mkdocs-section-index.
• The nag is not cosmetic noise from our own theme code — it's a print() call from properdocs.replacement_warning.setup(), invoked from a try/except ImportError block that these plugins' own __init__.py/plugin.py now carry (verified in mkdocs_redirects/plugin.py).
• It only fires when running from the real mkdocs executable, and can be silenced with DISABLE_MKDOCS_2_WARNING=true.
• It did not appear when installing the theme into a clean uv venv for the Python-3.9 smoke test — only in the environment with the full docs plugin stack from requirements-docs.txt.
• Did not install or run properdocs itself, and did not change any build tooling for this release — all release checks in RELEASING.md still used the documented mkdocs commands.

Why it needs a look (not urgent)

This smells like either a legitimate (if very aggressive) marketing push by a new properdocs fork coordinating with several popular MkDocs plugin maintainers, or a more concerning dependency-confusion/supply-chain move. Either way we should understand it before it affects contributors or CI:

• Is properdocs a legitimate, maintained fork, or a low-reputation/typosquat-style package?
• Which of our pinned plugin versions first introduced the properdocs.replacement_warning hook, and is it safe to keep floating on those plugins going forward?
• Should CI pin/avoid the affected plugin versions, or set DISABLE_MKDOCS_2_WARNING=true explicitly so a future, less benign payload doesn't ship unnoticed?
• Is there any indication of properdocs doing anything beyond printing the warning (e.g. during pip install)?

Suggested next steps

1. Check whether mkdocs-redirects, mkdocs-gen-files, mkdocs-section-index, mkdocs-literate-nav, mkdocs-rss-plugin changelogs mention this, and since which version.
2. Check the properdocs PyPI page / repo for reputation, maintainers, and download stats.
3. Decide whether to pin affected plugins below the version that added the hook, set DISABLE_MKDOCS_2_WARNING=true in CI/docs, and/or report concerns upstream.

Deliberately filed as post-release follow-up — does not block the 0.4.0 release.