Fixing all but auto-generated links in coordinates

This PR fixes all broken API links in coordinates (part of issue #1221) except the ones autogenerated using the package structure (and used as links for "bases" in the docs).

A few notes:
 - I couldn't make the link for astropy.units.UnitsError work, and removed the links. None of the ValueError or TypeError were linked, thus it was consistent to remove the UnitErrors' links too.

 - ``MatrixTransform`` was linked (broken link) and referred for further details, but I wasn't sure where it should be linked, didn't find an obvious location (nor looked it to be an obvious historical class that is now renamed/merged). Removed the links for this commit.

 - linked all ``None``, ``True`` and ``False`` to the Python docs.