Simple custom 404 page.

Github doesn't let us handle 404s within a project repository, so we
handle them at the organization level. In order to handle graceully for
specific projects, we're setting up redirects to projects' own 404.html.

Author: zpao

docs/404.md

@@ -0,0 +1,8 @@
+---
+layout: single
+title: Page Not Found
+---
+
+We couldn't find what you were looking for.
+
+Please contact the owner of the site that linked you to the original URL and let them know their link is broken.