Following thoughts lead to this pull-request:
1. now there is a place to collect information around federation.
2. mermaid handling inside of forgejo is more natural.
3. federation is not mirrored to code.forgejo, so I link to codeberg.
Close #1009.
Reviewed-on: https://codeberg.org/forgejo/docs/pulls/1055
Reviewed-by: Robert Wolff <mahlzahn@posteo.de>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>