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/reference/glossary/kubectl.md

@@ -4,16 +4,19 @@ id: kubectl
 date: 2018-04-12
 full_link: /docs/user-guide/kubectl-overview/
 short_description: >
-  A command line tool for communicating with a Kubernetes API server.
+  A command line tool for communicating with a Kubernetes cluster.
 
-aka: 
+aka:
+- kubectl
 tags:
 - tool
 - fundamental
 ---
- A command line tool for communicating with a {{< glossary_tooltip text="Kubernetes API" term_id="kubernetes-api" >}} server.
+Command line tool for communicating with a Kubernetes cluster's
+{{< glossary_tooltip text="control plane" term_id="control-plane" >}},
+using the Kubernetes API.
 
 <!--more--> 
 
-You can use kubectl to create, inspect, update, and delete Kubernetes objects.
+You can use `kubectl` to create, inspect, update, and delete Kubernetes objects.