Fix header being sticky to the top of the page

This also seems to fix the issue of the website being scrollable
horizontally on mobile devices.
2 jobs for 47-fix-header-not-staying-fixed-when-scrolling-on-mobile-browser in 5 minutes and 50 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed run_jekyll_latest_dependencies #70422

00:04:03

passed run_jekyll_production #70423

00:05:50