add mdx package

Author: dburles

packages/docs/src/pages/packages/index.mdx

@@ -13,6 +13,7 @@ In addition to the core library, Theme UI includes several optional packages.
 
 - [@theme-ui/core](/packages/core)
 - [@theme-ui/components](/packages/components)
+- [@theme-ui/mdx](/packages/mdx)
 - [@theme-ui/presets](/packages/presets)
 - [@theme-ui/prism](/packages/prism)
 - [@theme-ui/color](/packages/color)

packages/docs/src/pages/packages/mdx.mdx

@@ -0,0 +1,7 @@
+---
+title: "@theme-ui/mdx"
+---
+
+import Readme from '@theme-ui/mdx/README.md'
+
+<Readme />

packages/docs/src/sidebar.mdx

@@ -50,6 +50,7 @@
 - [Packages](/packages)
   - [@theme-ui/core](/packages/core)
   - [@theme-ui/components](/packages/components)
+  - [@theme-ui/mdx](/packages/mdx)
   - [@theme-ui/presets](/packages/presets)
   - [@theme-ui/prism](/packages/prism)
   - [@theme-ui/color](/packages/color)

packages/mdx/README.md

@@ -5,7 +5,7 @@ MDX component context for Theme UI
 
 https://theme-ui.com
 
-**Note:** *This package is included in the main `theme-ui` package, and generally should not be used on its own.*
+**Note:** *This package is included in the main `theme-ui` package and a separate installation is not required.*
 
 ```sh
 npm i @theme-ui/mdx