Link to documentation not relative to base path

Patch to fix the above mentioned issue.

Thanks @JoCowood I still very much dislike the preg_match() part of the code, which is kept. Shouldn't there be a core logic for this, as it's the same as for example for menu links?

We should clean that up, when touching the code here.

Also I think #3249606: How to make the cookie banner link to a page in the same language as the one being displayed on a multilingual site → is closely related.

For that reason, back to NW. Someone has to check the core or other contrib implementations for best practice handling of such (int/ext) link fields.