[TASK] EXT:documentation: Remove box around links

Since the base CSS has been updated to underline links on
mouse hover, the box that is shown in EXT:documentation
should be removed.

Resolves: #52935
Releases: 6.2
Change-Id: I385b703dc533ccfecf85aa86537bfed43ec2a995
Reviewed-on: https://review.typo3.org/24888
Reviewed-by: Sascha Egerer
Tested-by: Sascha Egerer
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind