There are a number of ways to do this. You can start with using XenForo's built-in page nodes, you could use a wiki style add-on (like Jaxel's), you could make your own add-on (duh lol), or you could use Kotomi Generic Script bridge. http://xenforo.com/community/resources/kotomi-generic-script-bridge.122/
But tbh that looks like a sort of wiki built outside of the forum software. All they did was copy the style and change the body from page to page.
I would personally use a wiki style add-on. I think there are a few options available in the resources.