Clarify ArgoCD + Longhorn steps: node vs cluster scope and expected output (#138)

[simlarsen]

Issue #138 reported the deploy steps were hard to follow. This PR clarifies the workflow without altering technical claims:

• Adds an upfront "How the pieces fit together" section distinguishing node-level prerequisites (open-iscsi/iscsid and the NFSv4 client, installed on every node's OS) from cluster-level resources (the ArgoCD Application, StorageClasses, Secret, RecurringJobs) applied once to the API server.
• States for each step whether it is committed to Git for ArgoCD to sync or applied directly with kubectl apply -f, and explains ArgoCD's Sync vs Health status and selfHeal.
• Adds realistic expected-output blocks (per-node environment check, kubectl get application longhorn -n argocd showing Synced/Healthy, kubectl get pods -n longhorn-system, kubectl get storageclass).
• Cross-links the companion Helm guide.

Facts verified against https://longhorn.io/docs/ , the Longhorn environment_check.sh source, and https://argo-cd.readthedocs.io/ . npm run validate passes.

Resolves #138