docs: Add @theme-ui/mdx to Gatsby install instructions (#2373)

Author: LekoArts

packages/docs/src/pages/getting-started/gatsby.mdx

@@ -8,7 +8,7 @@ To use Theme UI with [Gatsby][], install and use
 [`gatsby-plugin-theme-ui`](/packages/gatsby-plugin).
 
 ```sh
-npm i theme-ui gatsby-plugin-theme-ui @emotion/react @mdx-js/react
+npm i theme-ui @theme-ui/mdx gatsby-plugin-theme-ui @emotion/react @mdx-js/react
 ```
 
 Add the plugin to your `gatsby-config.js`.

packages/gatsby-plugin-theme-ui/README.md

@@ -3,7 +3,7 @@
 Gatsby plugin for adding Theme UI context
 
 ```sh
-npm i theme-ui gatsby-plugin-theme-ui @emotion/react @mdx-js/react
+npm i theme-ui @theme-ui/mdx gatsby-plugin-theme-ui @emotion/react @mdx-js/react
 ```
 
 ```js