Add a navbar example that uses <div> instead of <ul>

Created by: cvrebert

I hope we can agree that v3's requirement of using a ul.navbar-nav for nav links ended up being too restrictive and thus annoying. It complicates mixing other widgets (e.g. buttons, inputs) in with nav links, among other problems. Everything ought to use classes now, but it'd be good to include an example without a ul so as to both prove things work without uls and demonstrate the markup-independence to readers. CC: @mdo