analytics: Add back comment, possibly used by detection.

This might make it easier to upgrade to GA4.

docs/shared/tpl/googleanalytics.html

@@ -1,5 +1,5 @@
 [%- IF siteconfig.googleanalytics %]
-[%# https://developers.google.com/analytics/devguides/collection/upgrade/reference/gajs-analyticsjs %]
+<!-- Global site tag (gtag.js) - Google Analytics -->
 <script async src="https://www.googletagmanager.com/gtag/js?id=[% siteconfig.googleanalytics %]"></script>
 <script>
   window.dataLayer = window.dataLayer || [];