I've also had enough with various wikis. And at some point we didn't find a decent tool for documenting our projects. We decided to build our own tool https://nots.io (yay NIH syndrome). The main idea is to make possible to link any type of doc like plain or markdown text, files, google docs right with the code. To give all docs a clear scope. And make it easy to discover what code is documented from the site or IDE. We also track the relevance of each added document. When new commits come in and the code behind the doc changes, we decrease its relevance factor (we call it the fresh rate), and let you know if the document is obsolete now.

We have a demo (https://nots.io/demo). Check it out, I'd love to get feedback.