Update coordinates docs for APE5

This PR updates the coordinates documentation structure for the major APE5 changes.  There are a few places that are not fully updated, all marked with ``..todo ::``.  The ``frames.rst`` I will take care of (either in a later update to this PR or in a separate PR if we want to merge this now).  A few others are @taldcroft's purview  as they deal with `SkyCoord`, and the representations docs are probably better done by @astrofrog, @Cadair, or both.