[TASK] Make docheader finally embrace flexbox

The conversion to flexbox is now complete. Spacing is now
streamlined and hard limits for columns removed. SCSS-Variables
are converted to plain CSS-Variables. Compiling dependencies to
the Form and Dashboard module are now resolved.

Resolves: #99183
Releases: main
Change-Id: Icaeece4506654c13d20f8d79940e9c95c9424dbb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76809


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>