Add Elastic Agent to stack installation instructions (#2602) (#2603)

(cherry picked from commit 73420e4)

Co-authored-by: DeDe Morton <[email protected]>

Author: mergify[bot]

docs/en/install-upgrade/installing-stack.asciidoc

@@ -61,7 +61,7 @@ Install the Elastic Stack products you want to use in the following order:
 . Elasticsearch ({ref}/install-elasticsearch.html[install instructions])
 . Kibana ({kibana-ref}/install.html[install])
 . Logstash ({logstash-ref}/installing-logstash.html[install])
-. Beats ({beats-ref}/getting-started.html[install instructions])
+. Elastic Agent ({fleet-guide}/elastic-agent-installation.html[install instructions]) or Beats ({beats-ref}/getting-started.html[install instructions])
 . APM ({apm-guide-ref}/apm-quick-start.html[install instructions])
 . Elasticsearch Hadoop ({hadoop-ref}/install.html[install instructions])