Add documentation for federation overview (fixes #774)

[Nutomic]

Creating this PR on Github instead of Gitea as it seems relevant to a lot more people than just code.

This is roughly based on a suggestion from the SocialHub Forum. I made some adjustments to the format to avoid duplicate content, and to better describe how Lemmy works.

This document is targeted at people that are more or less familiar with ActivityStreams and ActivityPub protocols. It is meant provide a high-level overview of ActivityPub federation in Lemmy.

[dessalines]

A few things

[Nutomic]

Updated to address your comments, and I also rewrite the "federation philosphy" section.

I am also wondering whether we should add a more detailed explanation of how moderation works. As well as the instance blocklist/allowlist. But those might fit better into the administration docs.

Do you think its good to capitalise the terms "Community", "Comment" etc like this? And is there too much duplicate info, or any info missing?

[dessalines]

I am also wondering whether we should add a more detailed explanation of how moderation works. As well as the instance blocklist/allowlist. But those might fit better into the administration docs.

It seems good to me, we're still kinda working out federated moderation anyway. Capitalization is fine too, especially since they're concepts. I'll merge and we can refine it later if necessary.