Add links to Github, astropy.vo and pyvo on the top-level docs page

I think it would make sense for `astropy.vo`, `astroquery` and `pyvo` to cross-link to each other.
So as a first step, let's link to the others from astroquery?

I tried to describe in one sentence how `astroquery` compares to `astropy.vo` and `pyvo`, which might be incorrect. @keflavich Please check.

And I've included a link back to github because I like it when it's easy to browse between the docs and code/issues.