Replace 404.html with a Markdown page

Markdown is easier to maintain than HTML, and more in the spirit of
Jekyll.

Author: agude

404.html

@@ -0,0 +1,10 @@
+---
+layout: default
+title: "404: Page not found"
+permalink: "404.html"
+---
+
+# 404: Page not found
+
+Sorry, we've misplaced that URL or it's pointing to something that doesn't
+exist. [Head back home]({{ site.url }}) to try finding it again.