First hacky hacky hack hack at sphinx-book-theme

[Cadair]

At this point all this really does is two things:

1. Remove the need for our theme to have a conf.py because it's all done inside the entry point
2. Show that it's possible to use the pydata sphinx theme's header stuff as a way of templating out our own header.

Infrastructure todo (before we start dealing with styling):

• Add a global footer, and reconfigure the book theme article footer (I think we can do this the same way as the header).
• Add back copybutton.js (or is there a sphinx extension for that now?)
• Add back version.js
• Figure out the sunpy.org landing page?!