[TASK] Add targets file

Add index of labels to documentation for easier cross-referencing.

Resolves: #57787
Releases: 2.4
Change-Id: I3fabc5d8a1e74b5e746458cf7e72a1b057f01211
Reviewed-on: https://review.typo3.org/29321
Reviewed-by: Francois Suter
Tested-by: Francois Suter