fix(docs): use latest pymdownx + restore testing.md

Pin-free pymdownx gets latest version which fixes NoneType error
on bare code fences in Python 3.11.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs-src/Dockerfile

@@ -10,8 +10,8 @@ WORKDIR /docs
 
 RUN pip install --no-cache-dir \
     mkdocs==1.6.1 \
-    mkdocs-material==9.6.14 \
-    pymdown-extensions==10.14.3
+    mkdocs-material \
+    pymdown-extensions
 
 COPY mkdocs.yml /docs/
 COPY docs-src/ /docs/docs-src/