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.
It works but emits a warning:

```
checking consistency... /home/devel/pulp_rpm/docs/user-guide/troubleshooting.rst:: WARNING: document isn't included in any toctree
```

This part of the patch is to get rid of the warning.