These new `.in` and `.txt` are making the repository look very
cluttered. Move them to a separate `tools` directory to make it look a
bit cleaner.

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`.