[FLINK-5575][docs] in old releases, warn users and guide them to the latest stable docs
diff --git a/docs/_layouts/base.html b/docs/_layouts/base.html
index dfd0f65..37c8b94 100644
--- a/docs/_layouts/base.html
+++ b/docs/_layouts/base.html
@@ -57,6 +57,10 @@
+    <div style="position:fixed; bottom:0; left:0; z-index:99999; width:100%; text-align:center; padding:15px; border-top:1px dashed #CE4B65; background:#f6f0e3; font-weight:bold">
+       This documentation is for an out-of-date version of Apache Flink. We recommend you use <a href="https://flink.apache.org/q/stable-docs.html">the latest stable version</a>.
+    </div>
     {% comment %} Includes are found in the _includes directory. {% endcomment %}
     {% include navbar.html %}