[DOCS] Add documentation for system extension adminpanel

This change should allow users to:
*  Discover the extension via docs.typo3.org
*  Allow different target groups like administrator,
   integrator and editor to get an idea about the feature
   set and how to use the extension.

Resolves: #93260
Releases: master, 10.4
Change-Id: I90a8f356231cc49baf3c8fbedd9625300827c094
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68908


Tested-by: Daniel Siepmann <coding@daniel-siepmann.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>