Make Resource Types sticky past breakpoint

I'd love if the Resource Types documentation had a persistent navigation so I don't have to scroll to the top to look at the documentation for another resource. Here's my best attempt.

![screenshot](http://f.cl.ly/items/2J04233P3Z251R3q2S3l/Screen%20Shot%202014-07-15%20at%2010.18.33%20AM.png)

Two bugs that are beyond my UI skills:

* Page pops up when header becomes fixed. Solvable with an empty fixed-height element?
* Headers are hidden when navigating to them. We could make a different anchor for it to scroll to, but I didn't want to modify every doc page. See <http://nicolasgallagher.com/jump-links-and-viewport-positioning/demo/#method-D>.