doc: decouple sidebar scrolling

This lets the doc sidebar have its own scrolling container, making the
page easier to navigate in cases where previously the menu was scrolled
far off.

PR-URL: https://github.com/iojs/io.js/pull/1274


Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>