Refactor some common classes from docs css and examples, then prevent examples from using docs.css

jumbotron-narrow.html

@@ -31,6 +31,13 @@ title: Narrow page template
     padding-bottom: 19px;
   }
 
+  /* Custom page footer */
+  .footer {
+    padding-top: 19px;
+    color: #777;
+    border-top: 1px solid #e5e5e5;
+  }
+
   /* Custom container */
   .container-narrow {
     margin: 0 auto;
@@ -119,8 +126,6 @@ title: Narrow page template
     </div>
   </div>
 
-  <hr>
-
   <div class="footer">
     <p>&copy; Company 2013</p>
   </div>