Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
Yes, although I think you have to put it in `photutils` as well because we still support Astropy 0.4, no?

I did find the place where I saw this already ... please use it here, too:
https://github.com/astropy/photutils/blame/ff47d74a031a22c466ca60c11679be2a8197c45c/docs/photutils/index.rst#L169