build(docs): raise the mkdocs-material pin so the docs install on 3.12+

mkdocs-material 9.4.0 pins regex~=2022.4. That compatible-release bound caps
regex in the 2022 range, and no regex release in it ships a cp312 or cp313
wheel, so pip falls back to a source build that dies with
error: [Errno 2] No such file or directory: 'cc' on any machine without a C
compiler. The quickstart in README.md and about/running-tests.md both tell
people to install this file, so a current interpreter could not build the
docs at all.

Bumped to 9.7.7, the version the docs have actually been verified against.
Material dropped the upper bound in 9.5.0 (regex>=2022.4) and 9.7.x replaces
regex with backrefs entirely, so the pin no longer drags in an
unbuildable package.

Verified with a real install, not a resolver dry run -- uv's dry run happily
resolves the old pin because resolution is not buildability. Fresh 3.12 venv,
no compiler: pip install -r docs/requirements.txt exits 0 with regex absent
from the tree, and mkdocs build --strict against that venv exits 0.

Author: Archmonger

docs/requirements.txt

@@ -1,6 +1,6 @@
 mkdocs
 mkdocs-git-revision-date-localized-plugin
-mkdocs-material==9.4.0
+mkdocs-material==9.7.7
 mkdocs-include-markdown-plugin
 mkdocs-spellcheck[all]
 mkdocs-git-authors-plugin