$mdSidenav: Add docs

[epelc]

Currently $mdSidenav service is only partially documented through the md-sidenav example found here. However the example does not mention the isOpen() function and possibly other features(I havn't looked at the source yet). It also doesn't describe it's usage or even existence besides using $mdSidenav('component-id').toggle() several times in the example.

[epelc]

Just checked the source there is also a close() function. Also apparently it's documented but for some reason it doesn't show up on the website anywhere that I can find.

I don't know exactly how the docs work but we may just have to remove this line. I don't think it's meant to be private if it's used in several examples.

[ajoslin]

Related: #1311