docs: add outdated message (#84)

Fixes #83

Author: afuetterer

docs/mkdocs.yml

@@ -25,6 +25,7 @@ nav:
 
 theme:
   name: material
+  custom_dir: src/overrides
   features:
   - content.action.edit
   - content.action.view

docs/src/overrides/main.html

@@ -0,0 +1,8 @@
+{% extends "base.html" %}
+
+{% block outdated %}
+  You're not viewing the latest version.
+  <a href="{{ '../' ~ base_url }}">
+    <strong>Click here to go to latest.</strong>
+  </a>
+{% endblock %}