Change documentation urls

[ccurvey]

I think I got this right. This should fix the problems with the initial site homepage containing dead links. I think this will fix both #2030 and #2034 (if I understand the issues correctly)

[henri-hulski]

At the moment we automatically create the docs in the gh-pages branch.
So the solution would be to activate gh-pages in the settings and point mezzanine.jupo.org to it (see #2030).

Problem is that @stephenmcd is the person who has the permission and he is not responsive.

Alternatively we could set up an automatic update of readthedocs, but I don't know who has access here.

[ccurvey]

Until we can get the CNAME records, or the appropriate permissions for readthedocs, would it make sense to just point those links back to the .RST files in GitHub?

[damascene]

@ccurvey I just did a PR for that: #2047