Just to throw the idea out there, there's also the possibility of maintaining a documentation/wiki site using Jekyll (a static site generator), hosting the site on Github Pages, and have most editors write stuff in Markdown. This loses out on some dynamic power of wiki software like searching, but much of the time it isn't really necessary, and there are several benefits to using Jekyll in terms of structure, styling, and templating. Additionally it is by nature open-source, and being hosted on Github makes it decently reliable along with being able to leverage some of Github's features.
This option would obviously require a lot more investment to start up. I'm also hesitant on how the organization of admin/editors would go, as there isn't really a way to fence off only a portion of the codebase. The likely case would end up being that a group of people would be made collaborators ("admins") and public editing of the wiki would have to be done through pull requests -- this would also increase the barrier to entry to edit. Maybe something else could be set up to bridge the gap, dunno. The age groups brought in by DNH here are largely like high school and college-level and not too familiar with any of this stuff, and so wouldn't necessarily find editing easy to deal with. On the other hand, you could also just make tons of people collaborators and risk explosions. Leveraging Github issues for minor edits is also a possible option.
One example of this being used for this purpose is the
Scala documentation, which should pretty much speak for itself. Their
contribute page outlines some things that you might have to expect using this method.
Just for consideration. This feels like a risky but maybe profitable option.