Added simple web page, routes are in a separate file

templates/404.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+
+{% block title %}404{% endblock %}
+
+{% block content %}
+    <h1>404: Not Found</h1>
+    <p>
+        Go to the
+        <a href="/">main page</a>
+    </p>
+{% endblock %}