Add external link icon to learn chef link (chef#2361)

Signed-off-by: IanMadd <[email protected]>

Author: IanMadd

layouts/index.html

@@ -24,7 +24,7 @@ <h4><i class="far fa-map"></i>Getting Started</h4>
               <li><a href="/quick_start/">Quick Start</a></li>
               <li><a href="/platform_overview/">Platform Overview</a></li>
               <li><a href="/chef_overview/">Chef Infra Overview</a></li>
-              <li><a href="https://learn.chef.io/tracks/getting-started/">Learn Chef</a>
+              <li><a href="https://learn.chef.io/tracks/getting-started/" class="external">Learn Chef</a>
                 <div class="external_link_icon"></div>
               </li>
             </ul>