Design tweaks to remove the cluttered sidebar

The main goal was to remove the sidebar because I find it pretty hard to scan for doc links which is pretty much the only reason to ever go to the rust homepage after the initial "what's this about? ok cool let's use it!" phase.

I hope this makes sense. Happy to discuss things though. Here is a screenshot by the way:

![image](https://f.cloud.github.com/assets/183678/1204381/d7b7242a-2563-11e3-8062-572554271ab9.png)