[BUGFIX] Fix positioning of containers in EXT:styleguide BE module

The containers used in the backend module of EXT:styleguide are now
based on flex boxes and don't use absolute positioning anymore. This
enabled the possibility to scroll when the navigation needs more space
than the actual content and also fixes positioning in right-to-left
scenarios.

Resolves: #103794
Releases: main
Change-Id: Ie3fcbf7f7141bf16749dc66b45e185168e469260
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84147


Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Frank Nägler <frank.naegler@typo3.com>