Closes #13379.

Also, use Google's CDN per #11369.

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

- works when viewing page via file://
- sets a good example security-wise

...because we don't support it and it may cause problems for people
trying to view the docs in IE.

The sticky-footer example uses a drop down in its navbar, and dropdowns
require Bootstrap's JS to work.

Use the minified version of Bootstrap since this example is about the
CSS and not about the JS.

"nav-header" does not exist in Bootstrap. The class used for headers
in dropdowns is "dropdown-header".