Switch the base framework from Create React App to Next.JS

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/pages/404.tsx

@@ -0,0 +1,17 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import React from 'react'
+import { NotFoundErrorScreen } from '../components/common/routing/not-found-error-screen'
+
+/**
+ * Renders a hedgedoc themed 404 page
+ */
+const Custom404: React.FC = () => {
+  return <NotFoundErrorScreen />
+}
+
+export default Custom404