Currently the CI and the docs both have to duplicate the same inforation
about how to gather dependencies etc, and have to use hacky ways to run
them.

Add a new `docs` environment to our tox setup so that building docs is
as simple as running `tox -e docs`.

When we update our dependencies, developers will eventually need to
recreate their `tox` environments to get correct results. This happens
because `tox` isn't particularly good at recognizing changes in the
requirements files.