docs: switch from mkdocs to properdocs (#2946)

properdocs is a drop-in MkDocs fork by the original MkDocs maintainers.
The existing plugins (include-markdown, macros, intersphinx) and pymdown
extensions work unchanged; themes are unbundled, so the readthedocs
theme comes from properdocs-theme-readthedocs.

Assisted-by: ClaudeCode:claude-fable-5

.github/workflows/test.yml

@@ -23,7 +23,7 @@ on:
       - .travis.yml
       - README.md
       - azure-pipelines.yml
-      - mkdocs.yml
+      - properdocs.yml
       - noxfile.py
   workflow_dispatch:
     # allow manual runs on branches without a PR