Update the central dashboard customization documentation

[luntropy]

I would like to update the central dashboard customization documentation. When specifying URL for the external link, the key should be link instead of url.

Additional details can be found in this issue - kubeflow/kubeflow#7594

Details
Current example:

"externalLinks": [
        {
          "type": "item",
          "iframe": false,
          "text": "Kubeflow Website",
          "url": "https://www.kubeflow.org/",
          "icon": "launch"
        }

Working example:

"externalLinks": [
        {
          "type": "item",
          "iframe": false,
          "text": "Kubeflow Website",
          "link": "https://www.kubeflow.org/",
          "icon": "launch"
        }