Use venv to build documentation

The Sphinx documentation can be made to use the `program-output` plugin
which would invoke commands to generate inline documentation.

The resulting output should correspond to bash8 source, not the
currently installed version on the host running the doc.

That also let us ensure the requirements are properly defined.

Change-Id: Ie09707f35df1a5aa06995e31ba3160dcfb386b37
Merge "Use venv to build documentation"