docs: Improved visibility into viewing Juju unit logs

Hi,

Juju units logs can be viewed with:

```
sudo k8s kubectl logs --namespace openstack --container
```

This is currently documented under _How-to Guides_ > _Troubleshooting_ > _Inspecting the cluster_ > _Services hosted on Canonical Kubernetes_, which seems too deeply buried in the docs for how essential and common an operation it is. It is also difficult to search. Those of us with long experiences with Charmed OpenStack will try (and have already tried) to `juju ssh` to units then check under `/var/log/juju/`, which is empty in Sunbeam.

It seems reasonable to also document this in a much more prominent and easily searchable location, such as adding a section like _How-to Guides_ > _Operations_ > _Checking unit logs_.

Start in How-to Guides > Troubleshooting > Inspecting the cluster > Services hosted on Canonical Kubernetes and review how the existing kubectl logs command is documented. Add a prominent, searchable How-to Guides > Operations > Checking unit logs section with the shown command and its namespace, container, and pod placeholders; done when the guidance is easy to find in the docs.

Written by the indexing model from the issue text.