add 404 page

custom_theme/404.html

@@ -0,0 +1,7 @@
+{% extends "main.html" %} 
+
+{% block content %}
+    <h1>Page Not Found</h1> 
+    <p>The page you have requested was not found.</p>
+    <p><a href="/">Go back to the documentation home</a>.</p>
+{% endblock %} 

mkdocs.yml

@@ -76,6 +76,7 @@ theme:
   font:
     text: Roboto
     code: Roboto Mono
+  custom_dir: 'custom_theme'
 
 plugins:
   - mkdocstrings