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.

Start by locating the astropy.org pages for astroquery, montage-wrapper, and the affiliated-package registry that supplies their pointers. Review how package status is represented, then identify every affected page and verify that each pointer leads to the package website or its RTD documentation, with provisional packages handled according to their status.

Written by the indexing model from the issue text.