Follow the model of https://docs.codeberg.org/
The pages or sections that are not relevant for Forgejo (such as
working with the Codeberg CI are not included)
The pages that have not yet been adapted to Forgejo are set to link to
the Codeberg documentation instead
Since we have different types of documentation I consider it helpful to
have a one-liner explaining who we want to reach with each part.
Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>