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/generated/kubeadm_upgrade.md

@@ -1,10 +1,9 @@
 
-Upgrade your cluster smoothly to a newer version with this command.
+Upgrade your cluster smoothly to a newer version with this command
 
 ### Synopsis
 
-
-Upgrade your cluster smoothly to a newer version with this command.
+Upgrade your cluster smoothly to a newer version with this command
 
 ```
 kubeadm upgrade [flags]
@@ -12,43 +11,13 @@ kubeadm upgrade [flags]
 
 ### Options
 
-<table style="width: 100%; table-layout: fixed;">
-  <colgroup>
-    <col span="1" style="width: 10px;" />
-    <col span="1" />
-  </colgroup>
-  <tbody>
-
-    <tr>
-      <td colspan="2">-h, --help</td>
-    </tr>
-    <tr>
-      <td></td><td style="line-height: 130%; word-wrap: break-word;">help for upgrade</td>
-    </tr>
-
-  </tbody>
-</table>
-
-
+```
+  -h, --help   help for upgrade
+```
 
 ### Options inherited from parent commands
 
-<table style="width: 100%; table-layout: fixed;">
-  <colgroup>
-    <col span="1" style="width: 10px;" />
-    <col span="1" />
-  </colgroup>
-  <tbody>
-
-    <tr>
-      <td colspan="2">--rootfs string</td>
-    </tr>
-    <tr>
-      <td></td><td style="line-height: 130%; word-wrap: break-word;">[EXPERIMENTAL] The path to the 'real' host root filesystem.</td>
-    </tr>
-
-  </tbody>
-</table>
-
-
+```
+      --rootfs string   [EXPERIMENTAL] The path to the 'real' host root filesystem.
+```