Move kubectl overview to be section index

Also:
- use glossary definition in page introduction
- tidy broken link in What's Next section
- update links to refer to moved page

content/en/docs/concepts/cluster-administration/manage-deployment.md

@@ -154,7 +154,7 @@ deployment.apps/my-deployment created
 persistentvolumeclaim/my-pvc created
 ```
 
-If you're interested in learning more about `kubectl`, go ahead and read [kubectl Overview](/docs/reference/kubectl/overview/).
+If you're interested in learning more about `kubectl`, go ahead and read [Command line tool (kubectl)](/docs/reference/kubectl/).
 
 ## Using labels effectively