Added support for Markdown / MDX-style static pages.

[jeffgca]

MDSvex is a way to write svelte components using embedded markdown. See:

https://mdsvex.com/

This PR sets up the site's preprocessor to scan for .md files and use them as svelte components, eg:

<script></script>

# Title

Some text

* item 1
* item2

<style></style>