[TASK] Writing documentation

This is the first part of the documentation. Away further
parts to get documented in future.

Resolves: #30521
Releases: 6.0
Change-Id: I8af033d337808713e7a024c50226c0044b065602
Reviewed-on: https://review.typo3.org/24478
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer
Reviewed-on: https://review.typo3.org/24555