Wouldn't a wiki basically be exactly that?
I was trying to avoid naming any products on purpose, but, for example, there is a commercial product that is easy enough for most people and affordable that would work for this- and there are a few open source clones that come close.
The open source clones are not quite polished enough for non-technical users, but they are the closest thing I can think of, basically crossing over a wiki + Markdown for knowledgebase creation with Git-like versioning.