[BUGFIX] Use grid-gutter-width variable for margin

This fixes the alignment of the rows in the doc header and
the size of horizontal lines in dropdowns.

Replace the hardcoded 15px margin with the bootstrap grid-gutter-width
variable. The row margin override for .module-docheader-bar can be
removed completely.

Resolves: #94044
Releases: master
Change-Id: Iefa02fed9c00a31676d296e86e1bc75049810795
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68976


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Benni Mack <benni@typo3.org>