docs: Register the Juju controller, add more context

It should be stated that the
To run sunbeam `juju register-controller [..]`, the folder `~/.local/share/juju` must be empty
Else, sunbeam will complain about a conflict

Either we should document it
Or we should come with a way to merge the yaml file with exiting juju credentials with the new ones

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*Reported from: https://canonical-openstack.readthedocs-hosted.com/en/latest/how-to/misc/manage-external-juju-controllers/*