[TASK] Add a reST documentation

This documentation is far from being complete yet, but it should
be a starting point. Maybe other contributors will extend it in
some areas (like security, testing, documentation etc)

Resolves: #58259
Releases: 6.2

Change-Id: Ieb45ad3a4e0c0c6a1bd88bf73bc14009987512f7
Reviewed-on: https://review.typo3.org/29694
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen