A few updates to intersphinx

After some discussion with @bmbouter, we decided that this patch is a better
way to go for managing which docs version each branch points to for intersphinx
linking.
I expected this URL would point to the build docs. If this is in the other plugins it needs to be updated there too. See [this comment](https://github.com/pulp/pulp/pull/1321/files#r20525361).