PyKE docs need an "Other packages" page

[barentsen]

@gully How do you feel about adding your list of other data analysis packages (e.g. ranked by GitHub stars) as a page to the PyKE sphinx docs? The title of the page could be something like "Other packages", "See also" or "Third-party data analysis tools".

If we do this now, then Christina can include it in the screencast for her AAS splinter session talk. It would be good to flash it up in the screencast, because we don't want to give the impression that we think people should only use PyKE.