kubeadm: update the reference documentation for 1.15 (#14596)

* kubeadm-ref: update placeholder generated files

* kubeadm-ref: include reset-phase.md document

Also update reset.md to reflect the addition of reset phases.

* kubeadm-ref: update authored content

* kubeadm-ref: remove imports for removed "config upload" commands

* kubeadm-ref: add page for kubeadm upgrade phases

content/en/docs/reference/setup-tools/kubeadm/kubeadm-upgrade-phase.md

@@ -0,0 +1,26 @@
+---
+title: kubeadm upgrade phase
+weight: 90
+---
+In v1.15.0, kubeadm introduced preliminary support for `kubeadm upgrade node` phases.
+Phases for other `kubeadm upgrade` sub-commands such as `apply`, could be added in the
+following releases.
+
+## kubeadm upgrade node phase {#cmd-node-phase}
+
+Using this phase you can choose to execute the separate steps of the upgrade of
+secondary control-plane or worker nodes. Please note that `kubeadm upgrade apply` still has to
+be called on a primary control-plane node.
+
+{{< tabs name="tab-phase" >}}
+{{< tab name="phase" include="generated/kubeadm_upgrade_node_phase.md" />}}
+{{< tab name="control-plane" include="generated/kubeadm_upgrade_node_phase_control-plane.md" />}}
+{{< tab name="kubelet-config" include="generated/kubeadm_upgrade_node_phase_kubelet-config.md" />}}
+{{< /tabs >}}
+
+## What's next
+* [kubeadm init](/docs/reference/setup-tools/kubeadm/kubeadm-init/) to bootstrap a Kubernetes control-plane node
+* [kubeadm join](/docs/reference/setup-tools/kubeadm/kubeadm-join/) to connect a node to the cluster
+* [kubeadm reset](/docs/reference/setup-tools/kubeadm/kubeadm-reset/) to revert any changes made to this host by `kubeadm init` or `kubeadm join`
+* [kubeadm upgrade](/docs/reference/setup-tools/kubeadm/kubeadm-upgrade/) to upgrade a kubeadm node
+* [kubeadm alpha](/docs/reference/setup-tools/kubeadm/kubeadm-alpha/) to try experimental functionality