A couple of months ago, I wrote a Rails engine which provides applications with complete documentation library functionality by simply adding a gem.
Documentation is a Rails engine which provides a complete system for managing a set of hierarchical documentation. Once installed in an application, you’ll have a complete user interface for viewing as well as editing a set of markdown pages.
I’m already using this myself in a couple of places including the Viaduct docs and the new Sirportly docs.
Full documentation about how to get started can be found on GitHub and in the repo.