Document to_css_class as a supported internal

[ColumbusLabs]

What

Document datasette.utils.to_css_class() as a supported internal utility and register it with Datasette's @documented safeguard.

Why

Plugins already use this helper, but it was absent from the documented internals. That left its support status unclear and meant the documentation registry could not protect its API section from accidental removal.

Fixes #1743

Details

• describes unchanged inputs, ordered normalization, and collision-resistant suffixes
• includes examples matching the existing behavior tests
• uses the standard internals_utils_to_css_class anchor and decorator registration

Testing

• uv run pytest -n auto -m 'not serial' (2269 passed, 40 skipped, 6 xfailed, 15 xpassed)
• uv run pytest -m serial (36 passed)
• uv run pytest tests/test_docs.py::test_functions_marked_with_documented_are_documented
• uv run pytest tests/test_utils.py -k to_css_class
• warning-as-error Sphinx build
• Cog, blacken-docs, codespell, Black, and git diff --check