Document manual creation of kube config to access k8s proxy via PAT

The goal is to document how to manually craft a kube config that uses a PAT to access an agent cluster.

The kube config will looks something like this:

---
apiVersion: v1
kind: Config
clusters:
- name: gitlab
  cluster:
    server: https://kas.staging.gitlab.com/k8s-proxy
users:
- name: agent
  user:
    token: pat:<agent-id>:<pat-with-k8s_proxy-scope>
current-context: agent
contexts:
- name: agent
  context:
    cluster: gitlab
    user: agent