Adds demo link to navbar (#1624)

* Adds demo link to nav

* Updates CHANGELOG.md

* Apply suggestions from code review

* fix

Co-authored-by: Josh Izaac <josh146@gmail.com>

doc/xanadu_theme/header.html

@@ -28,6 +28,9 @@
         <a class="nav-link" href="https://pennylane.ai/qml">Quantum machine learning
         </a>
       </li>
+      <li class="nav-item">
+        <a class="nav-link" href="https://pennylane.ai/qml/demonstrations.html">Demos</a>
+      </li>
       <li class="nav-item">
         <a class="nav-link" href="https://pennylane.ai/install.html">Install</a>
       </li>
@@ -67,4 +70,4 @@
   </div>
   <!-- Collapsible content -->
 </nav>
-<!--/.Navbar-->
+<!--/.Navbar-->