Backbone has a great source demo: http://documentcloud.github.com/backbone/docs/backbone.html
The left side is the comments, and right is code. Is there any tool to generate such a site or pages?
The tool is called docco, by the main author of Backbone itself ;) It's a pretty minimal coffe script relying on markdown and pygments to do the job and there is a multitude of clones in other languages that do exactly the same thing if you don't like running node to generate documentation. You will find them in the same page.