Replace JavaScript for sticky sidebar navigation with CSS

[mischah]

Follow up of #754

I know that the navigation won’t be sticky for IE Users, but that’s not our target group, right?

So with using position: sticky, we could get rid of that:

https://github.com/yeoman/yeoman.github.io/blob/c44507e424ab51ffe080cb92c9f37f2296f78a76/app/assets/js/script.js#L11-L73