Make sticky docs 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
* Add waypoints sticky to project
* Rename global nav JS elements for better clarity
* Use Waypoints to properly display docs navigation

https://trello.com/c/KNHdE47Q

![middleman-waypoints-yay](https://cloud.githubusercontent.com/assets/816402/5020696/f4669a52-6a80-11e4-8eee-ab8e4453c83f.gif)

I would probably put this above in the `#toc` block like:

```
#toc {
  &.stuck { }
}
```