add 404 info

docs/overrides/404.html

@@ -0,0 +1,11 @@
+{% extends "main.html" %}
+
+<!-- Content -->
+{% block content %}
+  <h1>404 - Not found</h1>
+  <br>
+  FlatBuffers is currently migrating their documentation system and existing 
+  links may not work. 
+  
+  Please <a href="https://github.com/google/flatbuffers/issues/new?template=404-doc.md">file an issue</a> indicating the broken link.
+  {% endblock %}