docs: explain and recommend Robot Webservice Users

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Instead of using the Hetzner Account credentials for Robot, you can also create a separate webservice user in the Robot Console. This user is only able to access the webservice (API) and not log into the account.

This improves the security because your account credentials will not be exposed to the cluster.

This is also explained in the Webservice docs:

To be able to use the interface, a web service user is required. You can create this user in Robot via the user menu in the upper right corner under "Settings" -> "Web service and app settings".

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