message: [website] Fixes various Lighthouse errors/warnings
diff: -<html>
  +<!DOCTYPE html>
  +<html lang="en">
  <head>
  - <title>Sigma.js</title>
  <meta charset="UTF-8" />
  + <title>Sigma.js</title>
  + <meta
  + name="description"
  + content="a JavaScript library aimed at visualizing graphs of thousands of nodes and edges"
  + />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <link rel="apple-touch-icon" sizes="180x180" href="./assets/images/apple-touch-icon.png" />
  <link rel="icon" type="image/png" sizes="32x32" href="./assets/images/favicon-32x32.png" />
  <header>
  <input type="checkbox" id="toggle-menu" />
  <nav class="menu">
  - <label for="toggle-menu" tabindex="1">
  + <label for="toggle-menu" tabindex="0">
  <img src="./assets/images/icon-menu.svg" alt="Toggle menu" class="show-menu" />
  <img src="./assets/images/icon-close.svg" alt="Toggle menu" class="hide-menu" />
  </label>
  <h2>Go further</h2>
  <div class="contributing row">
  <div>
  - <h4>I want to know more</h4>
  + <h3>I want to know more</h3>
  <p>Look at <a href="https://github.com/jacomyal/sigma.js/blob/main/README.md">the documentation</a>.</p>
  </div>
  <div>
  - <h4>I have a problem</h4>
  + <h3>I have a problem</h3>
  <p>
  Ask your questions on <a href="https://stackoverflow.com/questions/tagged/sigma.js">StackOverflow</a>, or
  report bugs by
  </p>
  </div>
  <div>
  - <h4>I want to help</h4>
  + <h3>I want to help</h3>
  <p>
  Contributions are welcome! Reading
  <a href="https://github.com/jacomyal/sigma.js/blob/main/CONTRIBUTING.md">our contribution guide</a> is a
  