Simplify/streamline InternalURLs/ExternalURL situation
As we struggled with documenting the precise meaning of these configuration keys in #17667, it became clear that there is room for improvement here.
Starting from the documentation of the current state (https://doc.arvados.org/v2.3/admin/config-urls.html) and the ideas/code in #17667 (e.g. https://dev.arvados.org/issues/17667#note-11), come up with a plan for simplification and implement it.
The documentation will need to be updated as well.