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/setup/production-environment/tools/kops.md

@@ -231,7 +231,7 @@ See the [list of add-ons](/docs/concepts/cluster-administration/addons/) to expl
 ## {{% heading "whatsnext" %}}
 
 
-* Learn more about Kubernetes [concepts](/docs/concepts/) and [`kubectl`](/docs/reference/kubectl/overview/).
+* Learn more about Kubernetes [concepts](/docs/concepts/) and [`kubectl`](/docs/reference/kubectl/).
 * Learn more about `kops` [advanced usage](https://kops.sigs.k8s.io/) for tutorials, best practices and advanced configuration options.
 * Follow `kops` community discussions on Slack: [community discussions](https://github.com/kubernetes/kops#other-ways-to-communicate-with-the-contributors)
 * Contribute to `kops` by addressing or raising an issue [GitHub Issues](https://github.com/kubernetes/kops/issues)

content/en/docs/setup/production-environment/tools/kubeadm/create-cluster-kubeadm.md

@@ -500,7 +500,7 @@ options.
 * <a id="lifecycle" />See [Upgrading kubeadm clusters](/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade/)
   for details about upgrading your cluster using `kubeadm`.
 * Learn about advanced `kubeadm` usage in the [kubeadm reference documentation](/docs/reference/setup-tools/kubeadm/kubeadm)
-* Learn more about Kubernetes [concepts](/docs/concepts/) and [`kubectl`](/docs/reference/kubectl/overview/).
+* Learn more about Kubernetes [concepts](/docs/concepts/) and [`kubectl`](/docs/reference/kubectl/).
 * See the [Cluster Networking](/docs/concepts/cluster-administration/networking/) page for a bigger list
   of Pod network add-ons.
 * <a id="other-addons" />See the [list of add-ons](/docs/concepts/cluster-administration/addons/) to

content/en/docs/setup/production-environment/windows/user-guide-windows-containers.md

@@ -29,7 +29,7 @@ This guide walks you through the steps to configure and deploy a Windows contain
 control plane and a [worker node running Windows Server](/docs/tasks/administer-cluster/kubeadm/adding-windows-nodes/)
 * It is important to note that creating and deploying services and workloads on Kubernetes 
 behaves in much the same way for Linux and Windows containers. 
-[Kubectl commands](/docs/reference/kubectl/overview/) to interface with the cluster are identical. 
+[Kubectl commands](/docs/reference/kubectl/) to interface with the cluster are identical. 
 The example in the section below is provided to jumpstart your experience with Windows containers.
 
 ## Getting Started: Deploying a Windows container