Document federation requirements

[Tamschi]

This is a follow-up to #1463.

It would be good to document specific federation requirements for remote networks, i.e. that fed.brid.gy must be federated with in addition to bsky.brid.gy or web.brid.gy and (or?) individual custom domains of bridge accounts.

The explanation should mention 'limited federation mode' to make it easier to discover for Mastodon admins.

[snarfed]

Yes! This would be part of a bigger "I'm an instance admin, what do I need to know?" section that I want to write. Will do.