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.

(cherry picked from commit 2496757)

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.