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

pyproject.toml

@@ -68,7 +68,8 @@ docs = [
     "mkdocs-include-markdown-plugin==7.1.8",
     "mkdocs-intersphinx>=0.1.1; python_version >= '3.12'",
     "mkdocs-macros-plugin>=1.4.1",
-    "mkdocs==1.6.1",
+    "properdocs==1.6.7",
+    "properdocs-theme-readthedocs",
     "pymdown-extensions",
     "rich",
 ]