Add contributor documentation explaining how to run a single a11y test

[kaycebasques]

Is there a way to just run the single a11y test that I'm currently developing? I don't see any guidance on this: https://pydata-sphinx-theme.readthedocs.io/en/stable/community/setup.html#run-the-tests

[drammock]

In theory it can be done directly with pytest using a -k test_name_regex flag, assuming you're in a suitable environment. The nox setup is newish and I'm not very familiar with the tool, maybe @trallard can give better guidance?