Add substitutions for API URLs

Author: lcawl

docs/docset.yml

@@ -40,6 +40,8 @@ subs:
   ece:   "Elastic Cloud Enterprise"
   eck:   "Elastic Cloud on Kubernetes"
   ech:   "Elastic Cloud Hosted"
+  es-apis: https://www.elastic.co/docs/api/doc/elasticsearch/
+  es-serverless-apis: https://www.elastic.co/docs/api/doc/elasticsearch-serverless/
   es-issue:   "https://github.com/elastic/elasticsearch/issues/"
   serverless-full:   "Elastic Cloud Serverless"
   serverless-short:   "Serverless"

docs/reference/elasticsearch/rest-apis/search-rank-eval.md

@@ -31,8 +31,8 @@ This gives you a first estimation of your overall search quality, as well as a m
 
 For full details about the API syntax, refer to:
 
-* [{{es}} API documentation](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-rank-eval)
-* [{{es-serverless}} API documentation](https://www.elastic.co/docs/api/doc/elasticsearch-serverless/operation/operation-rank-eval)
+* [{{es}} API documentation]({{es-apis}}operation/operation-rank-eval)
+* [{{es-serverless}} API documentation]({{es-serverless-apis}}operation/operation-rank-eval)
 
 ## Examples