add 404 page

Author: Remi-Gau

docs/overrides/404.html

@@ -0,0 +1,14 @@
+{% extends "main.html" %}
+
+<!-- Content -->
+{% block content %}
+
+<div>
+    <img src="https://http.cat/404">
+</div>
+
+<h2 style="text-align: center;">
+    It seems that the page you are looking for does not exist.
+</h2>
+
+{% endblock %}