Added navigation role to example navbars

As per the docs:
http://getbootstrap.com/components/#navbar-default

  modified:   examples/carousel/index.html
  modified:   examples/jumbotron/index.html
  modified:   examples/navbar-fixed-top/index.html
  modified:   examples/navbar-static-top/index.html
  modified:   examples/navbar/index.html
  modified:   examples/non-responsive/index.html
  modified:   examples/starter-template/index.html
  modified:   examples/sticky-footer-navbar/index.html
  modified:   examples/theme/index.html