PG-407 Added a 404 page to docs

new file:   docs/overrides/404.html
	modified:   mkdocs.yml

docs/overrides/404.html

@@ -0,0 +1,9 @@
+{#-
+  This file was automatically generated - do not edit
+-#}
+{% extends "main.html" %}
+{% block content %}
+  <h1>404 - Not found</h1>
+  <p> 
+We can't find the page you are looking for. Try using the Search or <a href= "{{ config.extra.homepage | d(nav.homepage.url, true) | url }}"> return to homepage </a>. </p>
+{% endblock %}