This adds a step to the docs generation Makefile to generate
release.svg and snapshot.svg badges, modelled after the gitlab
badges.

This also adds the generated badges directory in docs to .gitignore

The sphinx_rtd_theme is a sphinx theme designed to look modern and be mobile-friendly
http://sphinx-rtd-theme.readthedocs.io/en/latest/index.html

Some of the warnings from sphinx-build are really just warnings,
but a lot of the things we want to avoid and really break documentation,
like broken internal references and some invalid rst directives should
really be errors.

Now we treat all warnings as errors, this should ensure that
any commits landing upstream never break the docs.

This is using the relatively new `sphinx-click` plugin and will
automatically update the documentation based on whatever changes
in the frontend.