Adjust docs so "[edit this page on github]" appears next to "Page Source"

This comes from #588 and a discussion with @kbarbary - the "[edit this page on github]" link in the astropy docscurrently appears in a somewhat awkward location.  See e.g., http://docs.astropy.org/en/latest/index.html , where it appears at the end of the text, but apparently in-paragraph.    It should be changed to instead be next to the "Page Source" link at the footer of the page. 

@mdboom does this seem fine to you (I think you added the code that adds the link in the first place)? 

If either @kbarbary or @mdboom think this can be done relatively quickly, it'd be nice to have this in v0.2.0, so the (permanent) docs for that version are fixed.

(Note that this link only appears on *non* auto-generated docs.)