Improve Meteor Docs navigation on small screen sizes

This adds a collapsing Table of Contents to small screen sizes, which otherwise is not shown. This makes the docs usable on small screens. This PR shouldn't affect larger displays.

It's a bit quick and dirty and I didn't add animations for the menu. But rendering the document is already a bit slow on my iPhone 5, so it didn't really seem worth it.

Fixes https://github.com/meteor/meteor/issues/1372 and per @glasser's comment does not affect how docs are written.