Clearly this isn't enough and we're working on proper Sphinx docs which should be ready in the next few days.
Extensibility: it's just a Django app, so you can include it in your existing project, or add your own apps / views / middleware etc. There are some hooks, e.g. for including your app into the main admin navigation - see https://github.com/torchbox/wagtail/blob/master/wagtail/wagt... for an example of where a hook gets called. It's a good candidate for developer docs!