docs(gh-pages): Update keyboard nav card to point at GitHub temporarily

Author: cpcallen

gh-pages/_index.html

@@ -286,7 +286,7 @@ <h4>
       </p>
     </div>
   </a>
-  <a class="card" href="plugins/keyboard-navigation/test/index.html">
+  <a class="card" href="https://github.com/google/blockly-keyboard-experimentation">
     <video autoplay playsinline muted loop width="195px">
       <source src="media/keyboard-navigation.mp4" type="video/mp4">
     </video>
@@ -297,6 +297,12 @@ <h4>
       <p>
         A plugin that allows users to manipulate and connect blocks using a keyboard-driven cursor.
       </p>
+      <p>
+	This plugin (and its demo) is temporarily unavailable while we
+	develop an new and improved version; you can follow along with
+	our work in the <code>blockly-keyboard-experimentation</code>
+	GitHub repository.
+      </p>
     </div>
   </a>
   <a class="card" href="plugins/shadow-block-converter/test/index.html">