Add discussion of Prefect worker access to `kube-system` and alternatives to K8s guide

[discdiver]

First check

• [X] I added a descriptive title to this issue.
• [X] I used GitHub search to find a similar request and didn't find it 😇

Describe the issue

Would be good to explain why Prefect worker needs access to kube-system and work arounds

From source code docs:

  There is no real unique identifier for a cluster. However, the `kube-system`
        namespace is immutable and has a persistence UID that we use instead.

        PREFECT_KUBERNETES_CLUSTER_UID can be set in cases where the `kube-system`
        namespace cannot be read e.g. when a cluster role cannot be created. If set,
        this variable will be used and we will not attempt to read the `kube-system`
        namespace.

Describe the proposed change

Add the above to the K8s guide doc and discuss

Alternatives to accessing via kube-system:

• Setting variable: PREFECT_KUBERNETES_CLUSTER_UID
• Helm chart for the worker accepts a value for clusterUid

Additional context

Somewhat related issue: #9851