More organization for guides

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). 


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Here's the structure I've got:

![image](https://cloud.githubusercontent.com/assets/2231765/3632189/7cfa4bc8-0ec1-11e4-9cb4-50c496ed922e.png)

Also, I added the "Help us improve..." to guides & api docs:

![image](https://cloud.githubusercontent.com/assets/2231765/3632259/c0c016ca-0ec2-11e4-85d0-90e0761db547.png)

I also made some style changes:

- Guides have the wide layout like API docs, not the narrow layout like "examples"
- Guide tables have the striped styles of API doc tables
- `li a` inside `.docs` and `.api-docs` _do_ get underlines. They were almost impossible to see!