Add ECK doc links to Heartbeat docs (elastic#20284) (elastic#20329)

Author: dedemorton

heartbeat/docs/running-on-kubernetes.asciidoc

@@ -4,6 +4,8 @@
 {beatname_uc} <<running-on-docker,Docker images>> can be used on Kubernetes to
 check resources uptime.
 
+TIP: Running {ecloud} on Kubernetes? See {eck-ref}/k8s-beat.html[Run {beats} on ECK]
+
 ifeval::["{release-state}"=="unreleased"]
 
 However, version {version} of {beatname_uc} has not yet been