[DOC] Category rootline in FE

Change-Id: I0772cb35313364d47ec6b356cfff7e8ebcdd458b
Resolves: #55312
Reviewed-on: https://review.typo3.org/28131
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
(cherry picked from commit d37ae279b6b95c9751edc751071487c05c048997)
Reviewed-on: https://review.typo3.org/28132