Right now our on-topic page in the Help Centre is quite sparse and out of date. Let's work together to create a new draft for what is on-topic, taking into account all the decisions that have been made in Meta over the years. The answer is community wiki, and I'll start with what we have already.
Our community previously expressed desire to update the Help Centre, but we never accomplished it. I also asked the community which related sites should we mention. I'm trying again now with a more collaborative approach. Once we've reach some kind of consensus, I'll remove the unresolved tag and replace it with needs-moderator as discussed here.