Add 'elasticsearch' crosslink to docset.yml (elastic#133491)

As per elastic/docs-content#2658 
Certain links to `docs-content` redirect/rewrite themselves back into documentation now living in `elasticsearch`.

To ensure these resolve this now ensures we fetch elasticsearch's own crosslinks. 

cc @leemthompo

Author: Mpdreamz

docs/docset.yml

@@ -13,6 +13,7 @@ cross_links:
   - docs-content
   - ecs
   - eland
+  - elasticsearch
   - elasticsearch-hadoop
   - elasticsearch-java
   - elasticsearch-js
@@ -112,4 +113,4 @@ subs:
   nlp:   "natural language processing"
   index-manage-app:   "Index Management"
   connectors-app:   "Connectors"
-  ingest-pipelines-app:   "Ingest Pipelines"
+  ingest-pipelines-app:   "Ingest Pipelines"