Raise warnings if a relative link is referenced with the external link directive #532
Description
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.