Add basic 404 page

Note how this is used in different server environments:

https://gohugo.io/templates/404/#automatic-loading

layouts/404.html

@@ -0,0 +1,9 @@
+{{ partial "header" . }}
+
+<h1>The page you're looking for isn't here</h1>
+
+<p>Sorry about that.</p>
+
+<p><a href="{{ "/" | relURL }}">Go to the home page</a></p>
+
+{{ partial "footer" . }}