docs: re-enable google analytics (#43974)

We recently switched to using the new ReadTheDocs with "addons". That includes its own
analytics, which is nice, but we also want to continue using our GA4 analytics.

Adding GA4 is no longer supported by RTD, so we have to add it manually.

- [x] re-add the gtag to all pages, manually

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/docs/_templates/layout.html

@@ -0,0 +1,12 @@
+{% extends "!layout.html" %}
+
+{%- block extrahead %} 
+  <!-- Google tag (gtag.js) -->
+  <script async src="https://www.googletagmanager.com/gtag/js?id=G-S0PQ7WV75K"></script>
+  <script>
+    window.dataLayer = window.dataLayer || [];
+    function gtag(){dataLayer.push(arguments);}
+    gtag('js', new Date());
+    gtag('config', 'G-S0PQ7WV75K');
+  </script>
+{% endblock %}