Add support for extlinks and update intersphinx config

This commit adds support for extlinks, which has been used to condense BZ links
as well as link to documents in platform.

The intersphinx config has been updated as well to be called `platform` instead
of `pulp`. The `rtd_builder` var has been added to `conf.py` to point to the
correct `objects.inv` file.

Some sphinx build warnings have also been addressed.