Add Gatsby doc to side bar

In the long term, we should probably add more specifics to this doc so we can link from .org. However, first step is to make it more discoverable.

Author: LB

packages/docs/src/sidebar.mdx

@@ -9,6 +9,7 @@
 - [API](/api)
 - [Theme Specification](/theme-spec)
 - [Customize](/customize)
+- [Theme UI and Gatsby](/getting-started/gatsby)
 - [Demo](/demo)
 - [Components](/components)
   - [Variants](/components/variants)