📝 (default.html): update preview link with backup and add rel attributes

Add a backup link to ensure content availability if the primary
preview link is down. Include `rel="nofollow noindex"` attributes
to prevent search engines from following or indexing these links,
which helps in managing SEO and controlling link behavior.

Author: MrHinsh

docs/_layouts/default.html

@@ -84,8 +84,7 @@
                                     </li>
                                     <li class="list-group-item">
                                         <i class="fab fa-twitter"></i>
-                                        <a href="https://preview.nkdagility.com/learn/azure-devops-migration-tools/{{page.url}}" class="ml-2" target="_blank">Preview</a>
-                                    </li>
+                                        <a href="https://preview.nkdagility.com/learn/azure-devops-migration-tools/{{page.url}}" rel="nofollow noindex" class="ml-2" target="_blank">Preview</a> (<a href="https://blue-river-093197403-preview.westeurope.5.azurestaticapps.net/{{page.url}}" rel="nofollow noindex" class="ml-2" target="_blank">backup</a>)</li>
                                 </ul>
                             </li>
                         </ul>