Update sidebar.mdx

Author: LB

packages/docs/src/sidebar.mdx

@@ -1,4 +1,6 @@
-- [Getting Started](/getting-started)
+- Getting Started
+  - [Getting Started](/getting-started)
+  - [Getting Started with Gatsby](/getting-started/gatsby)
 - [Theming](/theming)
 - [The `sx` Prop](/sx-prop)
 - [Styling MDX](/styling-mdx)
@@ -9,7 +11,6 @@
 - [API](/api)
 - [Theme Specification](/theme-spec)
 - [Customize](/customize)
-- [Theme UI and Gatsby](/getting-started/gatsby)
 - [Demo](/demo)
 - [Components](/components)
   - [Variants](/components/variants)