Added MG.net Google Analytics tag to docs site.

Author: tomspilman

templates/monogame/partials/theme.tmpl.partial

@@ -1,3 +1,13 @@
+<!-- Google tag (gtag.js) -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=UA-38431038-1"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'UA-38431038-1');
+</script>
+
 <script>
     const theme = localStorage.getItem('theme') || 'auto'
     document.documentElement.setAttribute('data-bs-theme', theme === 'auto' ? (window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light') : theme)