chore: fix 404 page (#133)

Author: alexandear

doc/app.yaml

@@ -10,4 +10,7 @@ handlers:
   upload: public/(.*)
 - url: /
   static_dir: public/
-
+- url: /.*
+  static_files: public/404.html
+  upload: public/404.html
+  error_code: 404

doc/layouts/404.html

@@ -1,5 +1,13 @@
 {{ define "main" }}
-	<h1>Page not found</h1>
+<header>
+	<span class="content">
+		<h1>Page not found</h1>
+	</span>
+</header>
+<main>
 
-	I'm sorry, but the requested page wasn’t found on the server.
+<div class="content">
+	<p>I'm sorry, but the requested page wasn’t found on the server. Try going back to the <a href="{{ "" | relURL }}">home page</a>.</p>
+</div>
+</main>
 {{ end }}