Improve visual design of documentation navigation

* Remove `border` from documentation navigation
* Add `margin` below `global-nav` so it's modular and in `content-container`
* Add margin to hero content in lieu of margin from global nav

https://trello.com/c/KNHdE47Q

![screen shot 2014-11-11 at 10 53 18 am](https://cloud.githubusercontent.com/assets/816402/4998928/d253b822-6991-11e4-80b5-dca5fa2107cd.png)

@carolanncan 

Good call!