Add support for extlinks

This commit adds support for extlinks, which has been used to condense BZ links
as well as link to documents in platform. Extlinks is preferred if you need to
link to an entire URL instead of to a specific ref.

Some sphinx build warnings have also been addressed.
I expect platformdoc to not hard code the URL.  I expect it to look up by label which allows the external sphinx to be in control of the URL.