[examples/data-literate][m]: factor our a working markdown+CSV (data literate) example from site (where we already had a demo).

* removed a few extraneous things
* added the README.md with some instructions

examples/data-literate/lib/markdown.js

@@ -0,0 +1,33 @@
+import matter from 'gray-matter'
+import toc from 'remark-toc'
+import slug from 'remark-slug'
+import gfm from 'remark-gfm'
+import footnotes from 'remark-footnotes'
+
+import { serialize } from 'next-mdx-remote/serialize'
+
+/**
+ * Parse a markdown or MDX file to an MDX source form + front matter data
+ *
+ * @source: the contents of a markdown or mdx file
+ * @returns: { mdxSource: mdxSource, frontMatter: ...}
+ */
+const parse = async function(source) {
+  const { content, data } = matter(source)
+
+  const mdxSource = await serialize(content, {
+    // Optionally pass remark/rehype plugins
+    mdxOptions: {
+      remarkPlugins: [gfm, toc, slug, footnotes],
+      rehypePlugins: [],
+    },
+    scope: data,
+  })
+
+  return {
+    mdxSource: mdxSource,
+    frontMatter: data
+  }
+}
+
+export default parse