fix(docs): pin the toolchain in a requirements file so pip caching works

actions/setup-python's pip cache needs a requirements.txt or
pyproject.toml to hash, and failed the whole job without one. Putting
the pin in a file rather than inline also puts it somewhere a person
would look for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

mkdocs.yml

@@ -75,3 +75,8 @@ markdown_extensions:
 
 plugins:
   - search
+
+# The pinned toolchain lives in docs/ so the Pages workflow can cache on it; it is
+# not a page.
+exclude_docs: |
+  requirements.txt