Raise warnings if a relative link is referenced with the external link directive

[dgarcia360]

Motivation

We noticed in https://github.com/scylladb/scylla/pull/11092/files that some documentation projects such as scylla-docs were using the external link directive for linking internal docs.

For example:

  * `Scylla Housekeeping and how to disable it <disable-housekeeping>`_

Instead of:

 * :doc:`Scylla Housekeeping and how to disable it <disable-housekeeping>`

While the link might work in the local environment, it might not work in production since the URL of the link changes due to multiversion.

Proposal

Raise a new warning if there are internal docs referenced using the external links directive.