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/kubectl/cheatsheet.md

@@ -423,7 +423,7 @@ kubectl get pods -A -o=custom-columns='DATA:spec.containers[?(@.image!="k8s.gcr.
 kubectl get pods -A -o=custom-columns='DATA:metadata.*'
 ```
 
-More examples in the kubectl [reference documentation](/docs/reference/kubectl/overview/#custom-columns).
+More examples in the kubectl [reference documentation](/docs/reference/kubectl/#custom-columns).
 
 ### Kubectl output verbosity and debugging
 
@@ -444,7 +444,7 @@ Verbosity | Description
 
 ## {{% heading "whatsnext" %}}
 
-* Read the [kubectl overview](/docs/reference/kubectl/overview/) and learn about [JsonPath](/docs/reference/kubectl/jsonpath).
+* Read the [kubectl overview](/docs/reference/kubectl/) and learn about [JsonPath](/docs/reference/kubectl/jsonpath).
 
 * See [kubectl](/docs/reference/kubectl/kubectl/) options.