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-reset-phase.md

@@ -0,0 +1,53 @@
+---
+title: kubeadm reset phase
+weight: 90
+---
+In v1.15.0, kubeadm introduces the `kubeadm reset phase` command with the aim of making kubeadm more modular. This modularity enables you to invoke atomic sub-steps of the reset process.
+Hence, you can let kubeadm do some parts and fill in yourself where you need customizations.
+
+`kubeadm reset phase` is consistent with the [kubeadm reset workflow](/docs/reference/setup-tools/kubeadm/kubeadm-reset/#reset-workflow),
+and behind the scene both use the same code.
+
+## kubeadm reset phase {#cmd-reset-phase}
+
+{{< tabs name="tab-phase" >}}
+{{< tab name="phase" include="generated/kubeadm_reset_phase.md" />}}
+{{< /tabs >}}
+
+## kubeadm reset phase preflight {#cmd-reset-phase-preflight}
+
+Using this phase you can execute preflight checks on a node that is being reset.
+
+{{< tabs name="tab-preflight" >}}
+{{< tab name="preflight" include="generated/kubeadm_reset_phase_preflight.md" />}}
+{{< /tabs >}}
+
+## kubeadm reset phase update-cluster-status {#cmd-reset-phase-update-cluster-status}
+
+Using this phase you can remove this control-plane node from the ClusterStatus object.
+
+{{< tabs name="tab-update-cluster-status" >}}
+{{< tab name="update-cluster-status" include="generated/kubeadm_reset_phase_update-cluster-status.md" />}}
+{{< /tabs >}}
+
+## kubeadm reset phase remove-etcd-member {#cmd-reset-phase-remove-etcd-member}
+
+Using this phase you can remove this control-plane node's etcd member from the etcd cluster.
+
+{{< tabs name="tab-remove-etcd-member" >}}
+{{< tab name="remove-etcd-member" include="generated/kubeadm_reset_phase_remove-etcd-member.md" />}}
+{{< /tabs >}}
+
+## kubeadm reset phase cleanup-node {#cmd-reset-phase-cleanup-node}
+
+Using this phase you can perform cleanup on this node.
+
+{{< tabs name="tab-cleanup-node" >}}
+{{< tab name="cleanup-node" include="generated/kubeadm_reset_phase_cleanup-node.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 alpha](/docs/reference/setup-tools/kubeadm/kubeadm-alpha/) to try experimental functionality