[FEATURE] Backend documentation module

The documentation module brings back access to extension manuals and
lets official TYPO3 documents (such as guidelines, tutorials and
references) as well as custom documents to be listed in a central place.

Resolves: #50486
Releases: 6.2
Change-Id: Ib35a48f72682844f1bfe569763a561a89e7d810b
Reviewed-on: https://review.typo3.org/22622
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers