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
 

content/en/docs/concepts/extend-kubernetes/api-extension/custom-resources.md

@@ -26,7 +26,7 @@ many core Kubernetes functions are now built using custom resources, making Kube
 Custom resources can appear and disappear in a running cluster through dynamic registration,
 and cluster admins can update custom resources independently of the cluster itself.
 Once a custom resource is installed, users can create and access its objects using
-[kubectl](/docs/reference/kubectl/overview/), just as they do for built-in resources like
+[kubectl](/docs/reference/kubectl/), just as they do for built-in resources like
 *Pods*.
 
 ## Custom controllers

content/en/docs/concepts/overview/kubernetes-api.md

@@ -23,7 +23,7 @@ The Kubernetes API lets you query and manipulate the state of API objects in Kub
 (for example: Pods, Namespaces, ConfigMaps, and Events).
 
 Most operations can be performed through the
-[kubectl](/docs/reference/kubectl/overview/) command-line interface or other
+[kubectl](/docs/reference/kubectl/) command-line interface or other
 command-line tools, such as
 [kubeadm](/docs/reference/setup-tools/kubeadm/), which in turn use the
 API. However, you can also access the API directly using REST calls.