docs: Add tip of old documents (#13960)

* docs: Add tip of old documents

* Update overrides/main.html

---------

Co-authored-by: Wang Xin <[email protected]>

Author: SWHL

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+{% block outdated %}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 %}