Documentation: Sphinx can build gh-pages ready

[penguinolog]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

See https://www.sphinx-doc.org/en/master/usage/extensions/githubpages.html for details

[penguinolog]

CNAME file content:

urwid.org