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/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