[docs] Add topic about stopping Elastic Agent (#19971) (#20164)

* Add topic about stopping Elastic Agent

* Add systemctl stop command

* Add kill command

* Add missing colon

x-pack/elastic-agent/docs/elastic-agent.asciidoc

@@ -22,6 +22,8 @@ include::install-elastic-agent.asciidoc[leveloffset=+1]
 
 include::run-elastic-agent.asciidoc[leveloffset=+1]
 
+include::stop-elastic-agent.asciidoc[leveloffset=+1]
+
 include::elastic-agent-command-line.asciidoc[leveloffset=+1]
 
 include::elastic-agent-configuration.asciidoc[leveloffset=+1]

x-pack/elastic-agent/docs/stop-elastic-agent.asciidoc

@@ -0,0 +1,43 @@
+[[stop-elastic-agent]]
+[role="xpack"]
+= Stop {agent}
+
+To stop {agent} and its related executables, stop the {agent} process. Use the
+commands that work for your system. 
+
+//TODO: Replace with tabbed panel when it's out of experimental phase.
+
+*Windows:*
+
+If you installed the Agent as a service, stop the service. If
+necessary, use Task Manager on Windows to stop {agent}. This will kill the
+{agent} process and any sub-processes it created (such as {beats}).
+
+*Linux or macOS:*
+
+Run the following command to get the ID of the `elastic-agent` process:
+
+[source,shell]
+----
+ps | grep elastic-agent
+----
+
+Then kill the process:
+
+[source,shell]
+----
+kill -9 PID
+----
+
+Where `PID` is the ID of the `elastic-agent` process.
+
+*Systemd:*
+
+The DEB and RPM packages include a service unit for Linux systems with systemd.
+On these systems, you can manage {agent} by using systemd commands. Use
+`systemctl` to stop the Agent:
+
+[source,shell]
+----
+systemctl stop elastic-agent
+----