[TASK] Link to extension manual

If an extension manual is available locally, it should be
linked in Extension Manager.

Resolves: #56880
Releases: 6.2
Change-Id: I24572348e0dc79513ece37477e3aee186676d923
Reviewed-on: https://review.typo3.org/28352
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Markus Klein
Tested-by: Markus Klein