Optional Matrix primer on /attend/existing

[strugee]

Someone I know does have a Matrix account, but hasn't used it in a while. While we were talking about the /attend flow it became clear that they could use a reminder of the basics of Matrix - e.g. "it's not quite federated, right?" was a question that came up.

[altsalt]

Hmm, would it make sense for this to be a sort of FAQ or are there specific edges/corners we want to focus on?