Add menu for How PKI works

Signed-off-by: yuexiao-wang <wang.yuexiao@zte.com.cn>

docs/swarm/how-swarm-mode-works/menu.md

@@ -16,3 +16,4 @@ weight=11
 
 * [How nodes work](nodes.md)
 * [How services work](services.md)
+* [How PKI works](pki.md)