Structure examples with appropriate <main>, <header>, <footer>

Structure examples with appropriate `<main>`, `<header>`, `<footer>`

docs/4.0/examples/navbar-top-fixed/index.html

@@ -42,13 +42,13 @@
       </div>
     </nav>
 
-    <div class="container">
+    <main role="main" class="container">
       <div class="jumbotron">
         <h1>Navbar example</h1>
         <p class="lead">This example is a quick exercise to illustrate how fixed to top navbar works. As you scroll, it will remain fixed to the top of your browser's viewport.</p>
         <a class="btn btn-lg btn-primary" href="../../components/navbar/" role="button">View navbar docs &raquo;</a>
       </div>
-    </div>
+    </main>
 
 
     <!-- Bootstrap core JavaScript