Add intersphinx and extlinks to platform

This links to pulp_{rpm,puppet,ostree,deb,docker}. It also adds support for
`:bz:` and `:fixedbugs:` links.
This name is going to collide for the pulp_python content type. I'll suggest either making this one 'pythonlang' or you could change the ones below  like "rpm" -> "pulp_rpm". I think renaming python -> pythonlang or something similar mining that its the docs for Python itself is cleaner.