feat: add very simple default theme

lib/theme/Docs.js

@@ -0,0 +1,8 @@
+import React from 'react';
+import Layout from '@theme/Layout';
+
+export default class Docs extends React.Component {
+  render() {
+    return <Layout>{this.props.children}</Layout>;
+  }
+}

lib/theme/Layout.js

@@ -0,0 +1,7 @@
+import React from 'react';
+
+export default class Layout extends React.Component {
+  render() {
+    return <div>{this.props.children}</div>;
+  }
+}

lib/theme/NotFound.js

@@ -0,0 +1,15 @@
+import React from 'react';
+import Layout from '@theme/Layout';
+
+export default class NotFound extends React.Component {
+  render() {
+    return (
+      <Layout>
+        <div>404 Page Not Found</div>
+        <div>
+          <img src="https://i.imgur.com/yG7L3Lb.gif" />
+        </div>
+      </Layout>
+    );
+  }
+}