Accessibility improvements for Material docs

This change covers a few accessibility fixes for the Material Design docs, including missing alt text, accessibility for the mobile menu button, an improved sidenav experience on page change, and a way to close the sidenav with the keyboard. A fuller-featured branch on `material-sidenav` is forthcoming, but at least wiring up the escape key made the docs sidenav easier to use. 
I like to have symmetry in my logic (for easier maintenance). Since the `backdrop` removes itself from the element parent, I think it should also add itself.

```js
var parent = $element.parent();

if (isOpen) {  
  backdrop.appendTo( parent ).on('click', onBackdropClick);
  parent.on('keydown', onKeyDown)
} else {
  backdrop.remove().off('click', onBackdropClick);
  parent.off('keydown', onKeyDown);
}
```