In order to ensure that feature additions don't break existing Flex
theme deployments, build the docs site on any branch push and PR.
To do so,
* Add a simple tox.ini that runs "pelican -s docs/pelicanconf.py" in
virtualenvs configured for Python versions 3.6 - 3.9.
* Add an additional GitHub Actions workflow that invokes these
testenvs with the matching Python version, and runs on any branch push
and on any pull request.
