Links to affiliated packages

For a few long standing affiliated packages there are pages under astropy.org (e.g. http://astropy.org/astroquery and http://astropy.org/montage-wrapper and possibly others).

I propose to remove these pages and use the pointers to either point to the websites of the packages listed in the registry, or to their RTD docs page. It can be a bit fine grained, too, e.g. packages with provisional status don't get it only when they are accepted with full status, etc.