Navigation header should be sticky in mobile view

**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.

Start by locating the navigation header and table-of-contents implementation for the spec website, then inspect its behavior in a mobile viewport. Done means the navigation header remains accessible after following a table-of-contents link, so returning to navigation does not require scrolling to the top.

Written by the indexing model from the issue text.