Add 404 page

Author: Andrei Jiroh Eugenio Halili

src/pages/404.js

@@ -0,0 +1,20 @@
+import React from 'react';
+import { Link } from 'gatsby';
+
+import Layout from '@rocketseat/gatsby-theme-docs/src/components/Layout';
+import SEO from '@rocketseat/gatsby-theme-docs/src/components/SEO';
+
+export default function NotFound() {
+  return (
+    <Layout title="Page not found!">
+      <SEO title="404: Not found" />
+      <p>You just gone to the wrong route. Did we just broke something?</p>
+      <p>
+        If everything is fine and you&#39;d like to go back to homepage, <Link to="/">click here</Link>
+        .
+      </p>
+      </br>
+      <p>But if you&#39;re in doubt, <Link to="https://github.com/code-server-boilerplates/docs/issues">file an new GitHub issue here</Link>.</p>
+    </Layout>
+  );
+}