Add share attribute for classic docs (#3221)

Author: karenzone

shared/attributes.asciidoc

@@ -182,6 +182,7 @@ Elastic Cloud
 :ecloud:      Elastic Cloud
 :esf:         Elastic Serverless Forwarder
 :ess:         Elasticsearch Service
+:ech:         Elastic Cloud Hosted
 :ece:         Elastic Cloud Enterprise
 :eck:         Elastic Cloud on Kubernetes
 :serverless-full:  Elastic Cloud Serverless