Factor out the code in tools/provision.py which installs a python2
and python3 venv into a module (tools/setup/setup_venvs.py) which
can also be used as a script.
Sphinx/RTD creates different anchor tags for subheadings than
Markdown does, and treats image embedding differently.
This commit updates hyperlinks to affected anchors,
and fixes an image embed for move to RTD.