"docs/coordinates/skycoord.rst":
- I added an example how to plot the data in an Aitoff projection with
  matplotlib.
"docs/coordinates/skycoord.rst":
- I updated the code accordingly to discussion in issue #2726 so that the
  doctests should work now.
- The pi is now also in math mode.
"docs/coordinates/skycoord.rst":
- Deleted pylab dependencies as suggested in issue #2726.
"docs/coordinates/skycoord.rst":
- Changes made as in pull request #2739. I inluded two examples: one simple (as
  before) and an additional one with more complexity.
Change wording in the new example.

closes #2739
Fix sphinx references to SkyCoord
Improve plot output for SkyCoord example
Merge pull request #3111 from hamogu/finish_2739

Example of plotting coordinates on an all-sky map