Add an affixed sidebar to documentation pages

Using the `{{sectionHeading}}` helper in your documentation will automatically add that title to the affix. If you don't use any `sectionHeadings`, then the affix won't be shown.

Pulled this from my Pure-Site branch:
yui/pure-site#202

![](http://f.cl.ly/items/0W1i460V2R3V1O3X340S/Screen%20Shot%202014-06-30%20at%204.51.54%20PM.png)