Sidebar navigation for package documentation.

Simple sidebar navigation. Also reformatted pkg.html so that it was easier to understand & parse.

This isn't merge ready: see below TODO, looking for feedback atm.

### TODO

 * Removing container width constraint makes non-package pages too wide. Wrap other pages in a new class?
 * scrollspy not _quite_ configured correctly, need to tune pixels-from-top to get it exaclty right
 * Should inspect & drill down into Introduction section