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.
Will this need to be updated to the specific version corresponding to the current version? If not then you would be browsing a specific version of the RPM docs and then when you click a link into platform you'll land on a different version.