Prep for the Documentation Sprint

I may have gone to town on this one a little hard.

What I did was deleted the whole of the documentation and started again. I tried to keep the git history of some files (the guide stayed I think).

I have moved this over to the astropy sphinx extensions, which the api docs make the most use of.

I fully expect that more of the old reference docs will make it back in, I just dropped them so I could start again.