Update EKS documentation for Auto Mode, Capabilities, and Atmos Auth

[cloudposse-internal]

What

Update EKS documentation to reflect the migration to EKS Auto Mode, as introduced by refarch-scaffold PR #897.

Documentation Changes

• EKS Foundational Platform — Rewritten for Auto Mode: removes eks/karpenter controller, eks/alb-controller, and eks/alb-controller-ingress-group references; describes Auto Mode managed compute/networking/storage; adds eks/ingress-class as replacement; adds migration callout with component mapping table
• EKS Capabilities — New section documenting managed Argo CD, ACK, and KRO capabilities with per-stage configuration (dev=all, staging/prod=argocd, core-auto=none)
• Atmos Auth for kubectl — Added atmos auth exec --identity plat-dev/terraform -- kubectl examples to the login docs
• Deploy Clusters guide — Updated step descriptions to reflect Auto Mode workflows (no manual Karpenter/ALB deploy)
• Vanity Domains tutorial — Updated Ingress examples with Auto Mode annotations (ingressClassName: alb, updated group names, annotation reference table)
• FAQ — Added legacy admonition to alb-controller-ingress-group question
• Design Decisions — Rewrote ingress controller decision for Auto Mode; added Auto Mode as recommended option in node pool architecture
• Platform page — Updated EKS description for Auto Mode
• Build fix — Restored missing examples/snippets/stacks/catalog/iam-role/example-app.yaml that was causing build failure

Components Removed from Docs

Old Component	Replacement
eks/karpenter (controller)	EKS Auto Mode manages nodes
eks/alb-controller	EKS Auto Mode manages ELB
eks/alb-controller-ingress-group	eks/ingress-class

Why

EKS Auto Mode simplifies cluster operations by letting AWS manage node groups, kube-proxy, coredns, vpc-cni, and ebs-csi-driver automatically. The documentation must reflect this architectural shift so users follow the correct setup path.

References

• Scaffold PR: https://github.com/cloudposse/refarch-scaffold/pull/897
• EKS Auto Mode docs