[DOCS] Add EOL notice. (#4255)

(cherry picked from commit 80e9b96)

Author: debadair

docs/page_header.html

@@ -0,0 +1,10 @@
+<p>
+  <strong>WARNING</strong>: Version 5.x has passed its 
+  <a href="https://www.elastic.co/support/eol">EOL date</a>. 
+</p>  
+<p>
+  This documentation is no longer being maintained and may be removed. 
+  If you are running this version, we strongly advise you to upgrade. 
+  For the latest information, see the 
+  <a href="../current/index.html">current release documentation</a>. 
+</p>