Align deployment docs and lifecycle gates

[igalshilman]

Summary

• align the README and guides around two required cluster stages plus an optional, separately state-managed service stage
• make the Terraform service readiness check generation-aware and document a safe generation-sensitive cluster rollout gate
• correct the operator 3.0.1 health reasons and add generation-aware Argo CD and Flux examples
• clarify reverse-order teardown, S3-versus-replicated metadata choice, zonal EBS maintenance capacity, and the SDK service contract
• soften the durability guidance while keeping the operational consequences explicit

Validation

• nix-shell --run ./scripts/validate.sh
• exercised scripts/wait-restatedeployment.sh with a stale Ready=True fixture and confirmed it waited for observedGeneration to reach the current generation

[pcholakov]

I have a few more niggles that I figured out earlier but I can file a follow-up with those.