The OP actually tells that story exactly. They love writing answers in slack precisely because they are ephemeral and targetted at a specific immediate need (which also makes it a lot easier than writing general-purpose documentation btw). But the persistent documentation "just decayed and eventually nobody used it." Exactly.
It's possible the right tools can make it easier to maintain (and thereby less likely to go out of date), but I don't think just simply taking a graph approach is sufficient or a very significant part of it. I'm not sure what that would be -- one obvious answer is tools that generate docs from code or otherwise tie it to code in such a way that when the code changes the docs are forced to change. But while that makes sense, every attempt at this I have seen results in disastrously unfriendly documentation (see eg "relish").
Other approaches are just to capture all that "answer the ephemeral question" documentation that is happening anyway in a way that makes it persistent and more easily searchable. A sort of "knowledge base" approach. (which, true, isn't tree-based). Some of the answers will be out of date when you find them later, but that is expected which somehow makes it less disruptive, and it's still better than nothing.