Documentation Link => Guides / Api

I see that "Examples" is our most common page (after `"/"`). I suspect people want an overview of the framework. 

This is low-hanging fruit for helping people get a soft introduction. Split the "Documentation" link into 2: "Guides" and "API". That way, people looking for conceptual info will know that there are guides there (and not be scared off by API docs).

Looks like this:

![guides_link](https://cloud.githubusercontent.com/assets/2231765/3692493/12b4911a-135d-11e4-8239-96e0355622ba.gif)