Example of plotting coordinates on an all-sky map

I'm going through old PRs that are still open. @dklaes already did a very good job on this in #2739 (Thank you for contributing to astropy!), but unfortunately he did not have time to finish that PR. I based this PR on his branch, so he will get full credit his contribution and I included the remaining comments in the the discussion in #2739 and shortened the text for the second example a little where it repeated the first.

CC: people who where involved in reviewing the original PR
@eteq @astrofrog @taldcroft @embray 

closes #2739
Updating this to `~astropy.coordinates.SkyCoord` should fix the warnings in the documentation build (there are a few other references to SkyCoord below that need to be fixed as well).