I stumbled upon this yesterday: http://www.manula.com/
It solves a pain point I’ve had for years for Poker Copilot - user docs that are not painful to manage. It does multiple versions so that I can keep Poker Copilot 4 docs AND Poker Copilot 5 docs around, with some content shared between them. It also does multi-language pretty well.
Best of all, it has no-hassle PDF generation, where it turns my user guide pages into a nice-looking PDF. THAT has been painful to do with WordPress, my solution for user guides until now.