This PR adds a sidebar menu when browsing the user guide of MLlib. It uses a YAML file to describe the structure of the documentation. It should be trivial to adapt this to the other projects.

![screen shot 2015-11-18 at 4 46 12 pm](https://cloud.githubusercontent.com/assets/7594753/11259591/a55173f4-8e17-11e5-9340-0aed79d66262.png)

Author: Timothy Hunter <timhunter@databricks.com>

Closes #9826 from thunterdb/spark-11835.