Gitea, GitHub and the like allow the owners of a repository to create an associated wiki by selecting a preference check box. You should be able to copy the relevant code from any free/open source implementation. It would be useful for providing Frequently Asked Questions for the community for example.
Wikipedia has its hierarchical categories, for an example that is already working. More historically, the librarians have the Dewey decimal system and the subject classification system.
But it’s a good idea and may give the fediverse an edge until reddit copies it.