Bring back 404 page

docs/pages/404.html

@@ -0,0 +1,6 @@
+---
+data:
+  title: Not found!
+path: /404.html
+template: not_found
+---

docs/templates/not_found.html

@@ -0,0 +1,9 @@
+{% capture main %}
+<main id="main">
+    <div>
+        <h1 id="title">Not found</h1>
+        <p>Oops! This page doesn't exist. Try going back to our <a href="{{ "/" | relURL }}">home page</a>.</p>
+    </div>
+</main>
+{% endcapture %}
+{% include "_default/baseof.html" %}