docs(federation): Document how to do local federation

[nickvergessen]

Fix #10686

🌏 Testing federation locally

When testing federated conversations locally, some additional steps might be needed,
to improve the behaviour and allowing the servers to talk to each others:

1. Allow self-signed certificates

   occ config:system:set sharing.federation.allowSelfSignedCertificates --value true --type bool
   occ security:certificates:import /path/to/the/nextcloud.crt
   occ security:certificates

2. Allow local servers to be remote servers

   occ config:system:set allow_local_remote_servers --value true --type bool

Additionally you can enable debug mode that will list local users as federated users options
allowing you to federate with accounts on the same instance. Federation will still work
and use the full federation experience and opposed to the federated files sharing not
create a local share instead.