[DOCS] Fix sidebar positioning

The sidebar should only be fixed once it's reached the top of the page.

Currently there are issues when viewing on a smaller screen. For example, I can't see the bottom half of the sidebar when viewing on my 11" laptop.

**Current sidebar positioning on my laptop:**

![current positioning](https://dl.dropboxusercontent.com/s/b4mqftmx5wle1m2/Screen%20Shot%202014-04-17%20at%2011.29.51%20PM.png)

**New positioning after scrolling down:**

![current positioning](https://dl.dropboxusercontent.com/s/ywse2ekebthqtsj/Screen%20Shot%202014-04-17%20at%2011.30.09%20PM.png)