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.
Consider updating the comment to refer to the builders agnostically. Something like: "This var sets the read the docs builder used by intersphinx when referring to other Pulp docs". For instance we don't use 'stable' on our builders. Also I don't think latest should ever be used except by master. Any real release should name the builder explicitly: ie:

```
rtd_build = '2.4-release'
```

I'm not sure how to make this more clear.