Move navbar into flow in carousel example

The carousel example works well for the single page demo, but the
styles don't work well for a multi-page site because the
.navbar-wrapper is 'position: absolute'. That requires every page
without a carousel apply padding to the body or to have enough content
to render below the .navbar.

This lets the .navbar be part of the flow and instead uses negative
margins to pull the .carousel underneath the .navbar. Pages that don't
have a .carousel require no extra styles to render beneath the .navbar
like usual.