Overhaul website

Hello,

The goal of this PR is to improve the clarity of the website. To do this I have:

- Split the website up into sections
- Redesigned portions of the website

You can view the site here: http://ember-cli-docs.divshot.io/

As it stands right now, the documentation reads like one very long README. This means that the docs for a first-time user who needs an introduction and the docs for an experienced user remembering an obscure addon hook are lumped together in one place. By splitting the docs into sections we can treat each audience in better isolation. Eventually I hope that this change will let us improve the “narrative” of each section to make reading the docs more enjoyable and more helpful.

I see this as a _work in progress_—if people generally see this as an improvement over the existing site I'll do whatever is needed to get it to completion. I did my best to separate the existing docs out into sections, but there is likely a better way to do it then I have done. I know there's some people out there with strong opinions on how docs should be, it would be great to hear from them.