[site,#572][l]: merge in data literate material.

* Nav bar and layout
* Main catch all route that loads material from content
* Data Literate: demo plus all associated components
  * content/data-literate/demo.mdx
  * components/* (pretty much all are related to demo)
  * lib/markdown.js, lib/mdxUtils.js
  * api/proxy.js to proxy remote urls (to handle CORs)
  * content/data-literate.md (DL home page - old data literate home page converted to markdown)
* excel-viewer.js: excel viewer demo from data literate
* package.json / yarn.lock
  * Nav: @headlessui/react @heroicons/react/outline @heroicons/react
  * CSV support for table: papaparse
  * Excel support for tables etc: xlsx
  * Vega: react-vega vega vega-lite
  * MDX: next-mdx-remote (yarn rm @next/mdx)

site/components/CustomLink.js

@@ -0,0 +1,16 @@
+import Link from 'next/link'
+
+export default function CustomLink({ as, href, ...otherProps }) {
+  return (
+    <>
+      <Link as={as} href={href}>
+        <a {...otherProps} />
+      </Link>
+      <style jsx>{`
+        a {
+          color: tomato;
+        }
+      `}</style>
+    </>
+  )
+}