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)

Sorry just saw this comment. The benefit would only be not having a qualifying class that removes a border, but to only put a border on the items that need it in the first place. So instead of removing something after it's there, telling it not be put there in the first place.

It's not really a big deal - just a thing. Not something I'd bring up usually but your css was so spotless!