Add explicit role="main" to <main> elements (twbs#22574)

For compatibility in IE10/IE11 which don't otherwise "know" this element's role/don't expose it to AT

Author: patrickhlauke

docs/examples/dashboard/index.html

@@ -91,7 +91,7 @@
           </ul>
         </nav>
 
-        <main class="col-sm-9 offset-sm-3 col-md-10 offset-md-2 pt-3">
+        <main class="col-sm-9 offset-sm-3 col-md-10 offset-md-2 pt-3" role="main">
           <h1>Dashboard</h1>
 
           <section class="row text-center placeholders">

docs/index.html

@@ -2,7 +2,7 @@
 layout: home
 ---
 
-<main class="bd-masthead" id="content">
+<main class="bd-masthead" id="content" role="main">
   <div class="container">
     <span class="bd-booticon outline">B</span>
     <p class="lead">Bootstrap is the most popular HTML, CSS, and JS framework in the world for building responsive, mobile-first projects on the web.</p>