Updates the PyFITS FAQ to serve as the astropy.io.fits FAQ

Also removed a few questions mostly related to installation that are already covered generally by the Astropy installation docs, and made some small improvements to other existing answers, including improving links to the API docs.

This brings to mind that maybe it would be good to have a general Astropy FAQ.  This would be different from the "known issues" page which specifically addresses known bugs and shortcomings.  But a FAQ that addresses common usage questions both in specific submodules and across Astropy (how to deal with warnings, logging, config, etc.) might be handy.  Most *answers* could be short, and link to the relevant documentation.  But I think it should be possible to come up with a fair number of actual FAQs.