Just wanting to be clear: you are looking for something to document your SaaS, not a SaaS that is for documentation, correct?
If the former, then you might take a look at the various static site generators out there such as jekyll, middleman, frank, and so on. They all let you create the html/css/js framework around your pages, even
being able to use what you have in Rails to maintain look and feel, and let your writers focus on content. Then it's all delivered up statically (read: FAST) by the server.