Yes, the problem is due to the ceph-dashboard charm not being promulgated (to make the deployment proceed by referencing the charm’s simple name). This is because the charm is in tech-preview (not stable).
The documentation you cited has now been updated. Sorry for the trouble.
The doc change is that the application, at its current state of development (for the next six months), should be deployed in this way:
juju deploy cs:~openstack-charmers/ceph-dashboard
Instead of what you tried:
juju deploy ceph-dashboard
There is also a change pending for the charm README.