Fix site build, add yarn.

site/config/next.config.js

@@ -1,10 +0,0 @@
-module.exports = {
-  async rewrites() {
-    return [
-      {
-        source: '/concepts/:page(.md)',
-        destination: '/concepts/:page',
-      },
-    ]
-  },
-}

site/next.config.mjs

@@ -35,6 +35,4 @@ const rewrites = async () => {
 }
 
 
-module.exports = {
+export default withMDX(rewrites(config))
-  withMDX(rewrites(config))
-}

site/package.json

@@ -10,6 +10,7 @@
     "@headlessui/react": "^1.4.1",
     "@heroicons/react": "^1.0.4",
     "@mdx-js/loader": "^2.0.0",
+    "@mdx-js/react": "^2.0.0",
     "@next/mdx": "^12.1.0",
     "@silvenon/remark-smartypants": "^1.0.0",
     "@tailwindcss/typography": "^0.5.2",

site/pages/_app.js

@@ -6,7 +6,6 @@ import { ThemeProvider } from 'next-themes'
 
 import '../styles/global.css'
 import siteConfig from '../config/siteConfig.js'
-import nextConfig from '../config/next.config.js'
 import Layout from '../components/Layout'
 import MdxPage from '../components/MDX'
 import * as gtag from '../lib/gtag'