update analytics in generate api docs site to use new UA4 (#136497)
Change the analytics instance that the generated api.flutter.dev and master-api.flutter.dev use.
diff --git a/dev/docs/analytics-header.html b/dev/docs/analytics-header.html
new file mode 100644
index 0000000..bed0852
--- /dev/null
+++ b/dev/docs/analytics-header.html
@@ -0,0 +1,7 @@
+<!-- Google Tag Manager -->
+<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f)
+})(window,document,'script','dataLayer','GTM-ND4LWWZ');</script>
+<!-- End Google Tag Manager -->
