Fix documentation build with Sphinx 9

[cananoo]

Fixes #200.

Sphinx 9 removed the deprecated add_javascript and add_stylesheet
application methods. Use the current add_js_file and add_css_file APIs in
the documentation configuration so h11 can build with current Sphinx releases.

Validation:

• python -m pytest -q -p no:ruff h11/tests (78 passed)
• Sphinx 9.1 configuration and documentation smoke build
• git diff --check

I used OpenAI Codex to assist with investigating and preparing this change,
and reviewed the resulting patch before submission.