Navigation header should be sticky in mobile view

[kfiven]

Link to problem area:
Navigation header on the spec website.

Issue
When clicking on any items in table of contents on mobile view, it takes you there but if you need to go back, then you'll have to scroll to the top.

[richvdh]

Worth noting that the nav header is sticky at widths above 768px.

This behaviour actually comes from the docsy theme that we use as a basis for styling the site.