Add an FAQ answer about whether Spacebar is federated

[jkulawik]

Is your feature request related to a problem? Please describe.
I cannot tell if I can chat with users registered on other Spacebar instances without testing it myself.

Describe the solution you'd like
Adding an FAQ section called "is Spacebar federated?" or something similar.

Describe alternatives you've considered
N/A

Additional context
Interoperability is an important feature when self-hosting is involved to avoid creating user "bubbles" and overwhelming free instance hosters.