feat: add Matomo analytics via Jekyll head-custom include

Adds _includes/head-custom.html with Matomo tracking script pointing to
analytics.synthossystems.com, and a minimal _config.yml to wire up the
minima GitHub Pages theme so the include is automatically loaded.

_includes/head-custom.html

@@ -0,0 +1,15 @@
+<!-- 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="//analytics.synthossystems.com/";
+    _paq.push(['setTrackerUrl', u+'matomo.php']);
+    _paq.push(['setSiteId', '1']);
+    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 -->