The `check` docs target now runs spelling, black, and lint, so all
current documentation quality checks can be run with a single command.
Also documented the lint-docs check's availability and usage.
This adds a `lint.py` script to run sphinx-lint on Django's docs files,
a mathing `lint` target in the `docs/Makefile` and `docs/make.bat`, and
updates `docs/requirements.txt` accordingly.
This adds:
- GitHub actions,
- tox configuration,
- pre-commit hook, and
- makefile rules
to run blacken-docs linter.
Co-authored-by: David Smith <smithdc@gmail.com>
