Update footer

[astrojuanlu]

Right now it says:

Built with Sphinx using a theme provided by Read the Docs.

Which probably comes from here:

https://github.com/readthedocs/sphinx_rtd_theme/blob/28c377f99a0fb8a6a7545ceaa599b7afd082d344/sphinx_rtd_theme/footer.html#L49-L58

It's not particularly handy to extend, but I think we should do something about it.