[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/pages/api/proxy.js

@@ -0,0 +1,26 @@
+import axios from 'axios'
+
+export default function handler(req, res) {
+  if (!req.query.url) {
+    res.status(200).send({
+      error: true,
+      info: 'No url to proxy in query string i.e. ?url=...'
+    })
+    return
+  }
+  axios({
+    method: 'get',
+    url: req.query.url,
+     responseType:'stream'
+    })
+  .then(resp => {
+    resp.data.pipe(res)
+  })
+  .catch(err => {
+    res.status(400).send({
+      error: true,
+      info: err.message,
+      detailed: err
+    })
+  })
+}