Add a "coordinates for developers" doc section #4913

Prompted by @mhvk's #4909 (comment) (prompted by a related comment by @astrofrog), this issue is about creating a section in the coordinates docs that explains some of the internals of the coordinates subpackage. This would be meant as a way to get potential contributors up-to-speed on some of the complexities and outright Python Sorcery that's involved in some of the more complex parts of coordinates.

This is probably a task for me, although it's possible @adrn, @taldcroft, or @astrofrog might want to at least start it as an exercise in trying to figure out what's the most confusing?