Add documentation for federation

[SystemKeeper]

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In Talk 19 we added federation support for chatting. Currently the documentation for admins and users is missing and should be added.

[Antreesy]

Merged docs for users: https://github.com/nextcloud/documentation/pull/11802

Do we need one for admins as well? If so, where do we place it?

[nickvergessen]

https://nextcloud-talk.readthedocs.io/en/latest/system-requirements/ has some administration sections