vault: add new nomad setup vault -check commmand (#19720)

The new `nomad setup vault -check` commmand can be used to retrieve
information about the changes required before a cluster is migrated from
the deprecated legacy authentication flow with Vault to use only
workload identities.

website/data/api-docs-nav-data.json

@@ -132,6 +132,10 @@
       {
         "title": "Snapshot",
         "path": "operator/snapshot"
+      },
+      {
+        "title": "Upgrade Check",
+        "path": "operator/upgrade-check"
       }
     ]
   },