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 is going to be a problem when we stop lock-stepping the versions for the different builders. I suggest removing the rtd_builder variable and its note. I also suggest the same in all the plugins conf.py.