Update navbar.html

Author: chienhsiang-hung

src/html/navbar.html

@@ -19,6 +19,7 @@
             <ul class="dropdown-menu">
               <li><a class="dropdown-item" href="css-visible.html">CSS Visible</a></li>
               <li><a class="dropdown-item" href="css-display.html">CSS Display</a></li>
+              <li><a class="dropdown-item" href="https://chienhsiang-hung.github.io/option-man/">CSS Display-Use Case</a></li>
             </ul>
           </li>
         </ul>