Add 404 Not Found page component and update routing and sitemap configuration

src/pages/NotFound404.js

@@ -0,0 +1,17 @@
+import React from 'react';
+import { Typography } from '@mui/material';
+import LegalPage from './LegalPage.js';
+
+const NotFound404 = () => {
+  const content = (
+    <>
+      <Typography variant="body1" paragraph>
+        This page is no longer available.
+      </Typography>
+    </>
+  );
+
+  return <LegalPage title="Page Not Found" content={content} />;
+};
+
+export default NotFound404;