More organization for guides

[Not ready for merge]

I think the guides are not-so-useful because their contents are not readily apparent. Also, they're so long that they're hard to navigate. I think a good solution would be to split them into categories then specialize them (like the Rails guides). 

Here's what I have in mind (I've mocked up "Principles" and "Models"):

![image](https://cloud.githubusercontent.com/assets/2231765/3567057/bec65c3e-0b0d-11e4-9460-62d07a6b119b.png)

I would also add a "Feedback" section which describes how people can contribute to the documentation:

![image](https://cloud.githubusercontent.com/assets/2231765/3567062/d98f0908-0b0d-11e4-8819-05b86eec39f8.png)

(Again, erm, "inspired" by the Rails guides.)


@pushrax do you also think this would be an improvement? If so, I'll press on!