docs(readme): clarify "Run the server" setup steps

[ShiriNmi1520]

What this PR does / why we need it:
This PR improves the README section 2. Run the server to make the setup actionable for users.

The previous text pointed to deploy/example.yaml but did not clearly describe what must be edited before applying it or how to verify deployment success. This caused confusion during initial setup.

This change adds:

• required pre-apply edits (replace placeholder IAM ARNs, set a unique clusterID, verify control-plane scheduling rules)
• concrete deploy/verification commands:
  • kubectl apply -f deploy/example.yaml
  • kubectl -n kube-system rollout status daemonset/aws-iam-authenticator
  • kubectl -n kube-system get pods -l k8s-app=aws-iam-authenticator
• clarification about where webhook kubeconfig is generated (/etc/kubernetes/aws-iam-authenticator/kubeconfig.yaml or the path configured via --generate-kubeconfig)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #270

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[k8s-ci-robot]

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[kmala]

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kmala, ShiriNmi1520

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