kubeadm: promote the "kubeadm certs" command to GA (#24410)

The command resided under "kubeadm alpha certs".
It will be promoted to GA in 1.20 as "kubeadm certs".

The existing command "kubeadm alpha" will
remain present for one more release, but it will be hidden
from documentation as it is deprecated.

content/en/docs/reference/setup-tools/kubeadm/generated/kubeadm_certs.md

@@ -0,0 +1,48 @@
+
+### Synopsis
+
+
+Commands related to handling kubernetes certificates
+
+### 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 certs</td>
+</tr>
+
+</tbody>
+</table>
+
+
+
+### 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>
+
+
+