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

noxfile.py

@@ -228,7 +228,9 @@ def docs(session: nox.Session) -> None:
     """
     pyproject = nox.project.load_toml()
     session.install("-e.", *nox.project.dependency_groups(pyproject, "docs"))
-    session.run("mkdocs", "serve" if session.interactive else "build", "--strict", *session.posargs)
+    session.run(
+        "properdocs", "serve" if session.interactive else "build", "--strict", *session.posargs
+    )
 
 
 @nox.session(default=False)