fixup! Replace link to doxygen with link to guides and fix link to deprecated doxygen pages

Author: LasseRosenow

_includes/header.html

@@ -21,6 +21,7 @@
               <li><a href="https://www.hackster.io/riot-os" target="_blank">[email protected]<i class="bi bi-box-arrow-up-right align-text-bottom"></i></a></li>
               <li><a href="{% link index.html %}#faq">FAQ</a></li>
               <li><a href="https://guide.riot-os.org/" target="_blank">Documentation <i class="bi bi-box-arrow-up-right align-text-bottom"></i></a></li>
+              <li><a href="https://doc.riot-os.org/" target="_blank">API Documentation <i class="bi bi-box-arrow-up-right align-text-bottom"></i></a></li>
             </ul>
           </li>
           <li class="dropdown">