Landscape: clarify in the example physcial configuration page that the landscpae instances can only be deployed externally to sunbeam

Canonical mattermost [thread](https://chat.canonical.com/canonical/pl/mawoqef8utgympda6uacfckush) (Sunbeam channel).

The [example physcial configuration page ](https://canonical-openstack.readthedocs-hosted.com/en/latest/reference/example-physical-configuration/) mentions landscape VMs. Readers could incorrectly imply that landscpae could be deployed by the sunbeam client (similar to observability)

Open the linked example physical configuration page and find the section mentioning Landscape VMs. Clarify there that Landscape instances must be deployed externally to Sunbeam and are not deployed by the Sunbeam client like observability components. Done means readers can no longer interpret the example as showing Sunbeam-managed Landscape deployment.

Written by the indexing model from the issue text.