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>

docs/requirements.txt

@@ -0,0 +1,5 @@
+# Pinned deliberately. The Material maintainers have announced that MkDocs 2.0
+# removes the plugin system and breaks every theme override with no migration
+# path, so an unpinned install would break this build on the day it lands — for a
+# docs site nobody was editing.
+mkdocs-material==9.7.7