message: [website] Precises demo iframe URL
diff: <section class="showcase">
  <div>
  - <iframe src="./demo/" allowfullscreen="true" width="100%"></iframe>
  + <iframe src="./demo/index.html" allowfullscreen="true" width="100%"></iframe>
  </div>
  <div class="overlay" onclick="this.remove()" tabindex="1">
  <h2 class="text-title">See sigma.js in action</h2>
  