specify expectations for "service DIDs" like feed generators

[bnewbold]

Eg, as documented at https://github.com/bluesky-social/feed-generator/tree/main

We haven't really done enough of these to spec these out. Not sure if we'll do a registry, or should try to encode these in Lexicons, or what. The current DID spec mentions that atproto_pds is required, but that isn't true for this; but on the other hand these aren't regular atproto accounts (they don't have a repo).