Added 404 page

assets/templates/404.phtml

@@ -0,0 +1,11 @@
+<?php partial('header', ['title' => 'PHP Apprentice']) ?>
+
+<div class="container center">
+    <a href="/" class="logo-404"><?= icon('elephant') ?></a>
+    <p class="message-404">Whoops! The page could not be found.</p>
+    <div class="toc-404">
+        <?php partial('table_of_contents') ?>
+    </div>
+</div>
+
+<?php partial('footer') ?>