Add Elasticsearch integration to list

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/8769

doc/integration/README.md

@@ -56,6 +56,8 @@ GitLab can be integrated with the following enhancements:
 - Configure [PlantUML](../administration/integration/plantuml.md) to use diagrams in AsciiDoc documents.
 - Attach merge requests to [Trello](trello_power_up.md) cards.
 - Enable integrated code intelligence powered by [Sourcegraph](sourcegraph.md).
+- Add [Elasticsearch](elasticsearch.md) for [Advanced Global Search](../user/search/advanced_global_search.md),
+  [Advanced System Search](../user/search/advanced_search_syntax.md), and faster searching.
 
 ## Integrations