Our company had this question raised recently. We've spent a day to gather our thoughts on existing documentation products and at the end of the day came up with a decision. We decided to use Slate (https://github.com/lord/slate). It's statically generated from markdown and can be deployed as a GitHub page. Though we placed it on our own NodeJS server as we needed some security around it.
I'm a fan of Sphinx + https://readthedocs.org . The best aspect from my point of view is that readthedocs will continue to present docs from older versions of your project, which is helpful to users who either can't upgrade yet to the latest or are in the process of upgrading.