message: [website] Fixes various LightHouse warnings
diff: <section class="showcase">
  <div class="wrapper">
  - <iframe src="./demo/index.html" allowfullscreen="true" width="100%"></iframe>
  + <iframe allowfullscreen="allowfullscreen" src="./demo/index.html" title="Sigma.js demo" width="100%"></iframe>
  <div class="overlay" tabindex="0">
  <h2 class="text-title"><span class="bg">See sigma.js in action</span></h2>
  <div>
  <a href="https://github.com/graphology/graphology-metrics">various metrics</a> or even
  <a href="https://github.com/graphology/graphology-communities-louvain">community detection</a>.
  <strong>
  - You can see an overview <a href="https://graphology.github.io/standard-library.html">here</a>.
  + You can see an overview
  + <a href="https://graphology.github.io/standard-library.html">in the documentation</a>.
  </strong>
  </p>
  </li>
  