Add subheadings for readability

modules/manage/pages/cluster-maintenance/cluster-diagnostics.adoc

@@ -28,6 +28,8 @@ See the xref:reference:rpk/rpk-cluster/rpk-cluster-self-test.adoc[`rpk cluster s
 
 == Self-test command examples
 
+=== Start self-test
+
 To begin using self-test, run the `self-test start` command.
 
 ----
@@ -49,6 +51,8 @@ rpk cluster self-test status
 
 The `self-test start` command returns immediately, and self-test runs its benchmarks asynchronously.
 
+=== Check self-test status
+
 To check on the status of self-test, run the `self-test status` command.
 
 [,bash]
@@ -86,6 +90,8 @@ include::reference:partial$rpk-self-test-descriptions.adoc[]
 .Example status output: test results
 include::reference:partial$rpk-self-test-status-output.adoc[]
 
+=== Stop self-test
+
 To stop a running self-test, run the `self-test stop` command.
 
 ----

modules/reference/pages/rpk/rpk-cluster/rpk-cluster-self-test-status.adoc

@@ -17,7 +17,7 @@ Node 1 is still running net self test
 * No jobs running
 ** Returns the cached results for all brokers of the last completed test.
 
-include::reference:partial$rpk-self-test-status-descriptions.adoc[]
+include::reference:partial$rpk-self-test-descriptions.adoc[]
 
 == Usage