[DOCS] Add details about how to extend the admin panel

This commits adds the following hints:
* module classes needs to be set to public
* module classes needs to implement the ShortInfoProviderInterface
* description on how to add custom page settings
* description on how to add custom module settings

Resolves: #104242
Releases: main, 12.4
Change-Id: Id495359e007bf09e38a80bc1097c95946f964217
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84984


Tested-by: core-ci <typo3@b13.com>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>