Observability: clarify where the cos cluster is deployed

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

The emebedded observability is deployed in the cloud controller node. The [example physcial configuration page ](https://canonical-openstack.readthedocs-hosted.com/en/latest/reference/example-physical-configuration/) mentions observability VMs so readers could incorrectly assume these VMs are for embedded observability (similar to the fact that the sunbeam controller is outside the cloud controller node) instead of being an external cos cluster

Start with the example physical configuration page linked in the issue and review its observability VM description alongside the canonical thread. Clarify that embedded observability runs on the cloud controller node, while the observability VMs refer to an external COS cluster; done when readers cannot confuse these deployments.

Written by the indexing model from the issue text.