[SPARK-12324][MLLIB][DOC] Fixes the sidebar in the ML documentation

This fixes the sidebar, using a pure CSS mechanism to hide it when the browser's viewport is too narrow.
Credit goes to the original author Titan-C (mentioned in the NOTICE).

Note that I am not a CSS expert, so I can only address comments up to some extent.

Default view:
<img width="936" alt="screen shot 2015-12-14 at 12 46 39 pm" src="https://cloud.githubusercontent.com/assets/7594753/11793597/6d1d6eda-a261-11e5-836b-6eb2054e9054.png">

When collapsed manually by the user:
<img width="1004" alt="screen shot 2015-12-14 at 12 54 02 pm" src="https://cloud.githubusercontent.com/assets/7594753/11793669/c991989e-a261-11e5-8bf6-aecf3bdb6319.png">

Disappears when column is too narrow:
<img width="697" alt="screen shot 2015-12-14 at 12 47 22 pm" src="https://cloud.githubusercontent.com/assets/7594753/11793607/7754dbcc-a261-11e5-8b15-e0d074b0e47c.png">

Can still be opened by the user if necessary:
<img width="651" alt="screen shot 2015-12-14 at 12 51 15 pm" src="https://cloud.githubusercontent.com/assets/7594753/11793612/7bf82968-a261-11e5-9cc3-e827a7a6b2b0.png">

Author: Timothy Hunter <timhunter@databricks.com>

Closes #10297 from thunterdb/12324.