Allow internal links in Documentation URL

[gavindsouza]

I'd like to be able to not hardcode hostname as current site domain could change anytime in the future. However, adding a link like /docs/some-page - essentially anything without a hostname throws a "Not a valid URL" error and

[barredterra]

I think this applies to all URL fields and attaching a URL as a file

[ankush]

This can be relaxed for doc links for sure. Don't see a point in validating this aggressively, it can just be a warning instead of error.

[bosue]

Relative URLs should be a configuration option.

On some sites / pages / in some fields you might want to enforce absolute URLs.
In other contexts you might want to allow only relative URLs.

For a couple of reasons, including security and integrity, the latter case seems even more important.