feat: add matomo support

[andrea-dintino]

Test implementation of Matomo tracking (not sure if it works!),

• using https://github.com/datagouv/vitepress-plugin-matomo
• and https://www.npmjs.com/package/@datagouv/vitepress-plugin-matomo

Matomo tracking script is:

<!-- Matomo -->
<script>
  var _paq = window._paq = window._paq || [];
  /* tracker methods like "setCustomDimension" should be called before "trackPageView" */
  _paq.push(['trackPageView']);
  _paq.push(['enableLinkTracking']);
  (function() {
    var u="https://matomo.dyne.org/";
    _paq.push(['setTrackerUrl', u+'matomo.php']);
    _paq.push(['setSiteId', '9']);
    var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
    g.async=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s);
  })();
</script>
<!-- End Matomo Code -->