feat(samples): improve simple website sample

If the site doesn't load properly a warning is shown.

effective-shell-playground/websites/simple/code.js

@@ -1,3 +1,10 @@
+//  This function changes the header text for the webpage.
 function changeHeader(newHeader) {
   document.getElementById("header").innerText = newHeader;
 }
+
+//  When the webpage is loaded...
+window.addEventListener('DOMContentLoaded', function() {
+  //  ...hide the warning message.
+  document.getElementById("warning").display = 'none';
+});

effective-shell-playground/websites/simple/index.html

@@ -14,8 +14,12 @@
         <h1 id="header">My New Project</h1>
       </header>
       <section>
-        <p>This is a sample webpage, with some JavaScript for logic and CSS.</p>
+        <p>This is a simple web page. The <code>index.html</code> file defines the content, the <code>styles.css</code> file defines the styles and the <code>code.js</code> file makes the website interactive!</p>
         <button onclick="changeHeader('My Even Newer Project')">Update Title</button>
+        <div id="warning" style="color: red">
+          <h2>Warning</h2>
+          <p>If you see this message then the site is not loading properly - maybe you have disabled JavaScript in your browser?</p>
+        </div>
       </section>
     </body>
 </html>