[TASK] Automatic Intersphinx mapping (extended)

Extend feature #52347 to support custom (USER) documents as well.

Change-Id: I6b376514b6520e6eba2daea4b576563e01d25946
Relates: #52347
Resolves: #54736
Reviewed-on: https://review.typo3.org/26637
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers