Match specificity in carousel example

".carousel img", the selector from the 2.3.x branch, now has lower
specificity than the new carousel, which uses ".carousel-inner > .item
> img". That means the new styles win and set "height: auto;", but the
example needs "height: 500px;".

Match the selector specificity to make the example's styles win.
Let .navbar flow in carousel example

The carousel example works well on the singe page, but because the
.navbar-wrapper is "position: absolute;" it is not a good example to
follow for a site without a carousel on every page.

Moving the negative margin from the .navbar-wrapper to the .carousel
and putting the navbar in the flow lets the .navbar-wrapper work on
pages without the carousel.