Migrate documentation to MkDocs proper

This is still a bit of a work in progress, but throwing it up so people can see and provide feedback if they want.
Oh, so it is. Why isn't it here then? https://github.com/tomchristie/django-rest-framework/blob/master/mkdocs.py#L84

I'm amazed that I managed toguess exactly the right place below the 2.4 annoucement.