well there are wikis, but that may be a bit overkill, and there aren’t many free wiki hosters.

perhaps more suitable would be collaborative editing/note-taking platforms. there are the open source ones like etherpad and hedgedoc, where you can choose your own instance (instance list for etherpad and hedgedoc). there’s also notion, which I think the admin team here uses, so you can ask them for more info, but it’s a centralized platform.