layouts/404.html: Add minimal not found page

You still need to instruct your web server to use this. See the Hugo
documentation for more information: https://gohugo.io/templates/404/

Closes #72

layouts/404.html

@@ -0,0 +1,12 @@
+{{ define "main"}}
+
+<article class="blog-post">
+  <header>
+    <h2 class="blog-post-title">Page Not Found</h2>
+  </header>
+  <p>Page not found. Go back <a href="{{ "/" | relURL }}">home</a>.</p>
+</article> <!-- /.blog-post -->
+
+{{ end }}
+
+{{- /* vim: set ts=2 sw=2 et: */}}