We used mailing list per topic (think channels) that anyone could subscribe to, and piper mail for searchable archives, and a $100 bounty for every relevant topic turned into a GFM document (with guidelines for minimal structure w.r.t. pros and cons and whats, whys and hows). Seemed to work. Of course this was before people discovered Slack, destroyer of institutional knowledge.
I'm actually kind of excited about the idea of an internal StackOverflow. Of course, people would have to actually use it, and that battlefield is littered with the shattered corpses of a million productivity and communication and social tools.