Add esre-ref attribute (#3162)

Author: lcawl

shared/attributes.asciidoc

@@ -130,6 +130,7 @@ Elastic-level pages
 :integrations-devguide: https://www.elastic.co/guide/en/integrations-developer/current
 :time-units:           {ref}/api-conventions.html#time-units
 :byte-units:           {ref}/api-conventions.html#byte-units
+:esre-ref:             https://www.elastic.co/guide/en/esre/{branch}
 
 //////////
 APM agents