Split credits and license

This splits the ``creditsandlicense.rst`` file into an author list and a license page that are separate.  This probably makes sense to do on its own, anyway, but it's intended to prepare for a PR on astropy-website that will change the "The Team" link to point to the docs.  That way we only have to maintain one contributor list instead of two.

It also slightly reformats both, particularly to make the author list look a bit more like the "The Team" page.  It should have all the same people in it, though.