adds some events to google analytics when 404 is found (#1765)

Author: nhayfield

src/theme/NotFound/Content/index.tsx

@@ -0,0 +1,34 @@
+import React, {JSX, useEffect} from 'react';
+import Content from '@theme-original/NotFound/Content';
+import type ContentType from '@theme/NotFound/Content';
+import type {WrapperProps} from '@docusaurus/types';
+
+type Props = WrapperProps<typeof ContentType>;
+
+export default function ContentWrapper(props: Props): JSX.Element {
+
+  useEffect(() => {
+    if (typeof window !== 'undefined' && window?.gtag) {
+      //normal page_view event
+      gtag("event", "page_view", {
+        page_title: "404 Not Found",
+        page_path: window.location.pathname,
+        event_category: "Errors",
+        event_label: "404"
+      });
+
+      //custom event that might be easier to work with
+      window.gtag("event", "not_found", {
+        page_title: "Page Not Found",
+        page_path: window.location.pathname,
+        event_category: "Errors",
+        event_label: "404"
+      });
+    }
+  }, [])
+  return (
+    <>
+      <Content {...props} />
+    </>
+  );
+}