Document controller-backed E2E and cluster CNI

[bcho]

Related PRs

• Add controller-backed E2E machine state #233 — controller-backed E2E machine state
• Remove legacy bridge CNI setup and use Unbounded-Net in E2E #234 — Unbounded-Net CNI migration (this PR's base)

This PR is intentionally based on hbc/unbounded-net-cni-e2e; review only the documentation and demo-asset diff from that branch.

Summary

• document the ConfigMap-backed in-cluster machine endpoint
• replace local-file E2E machine-state references with controller-backed state
• document controller-driven repave validation
• document the ARM-compatible machine response, ETag drift version, and separate status patch model
• document the no-CNI AKS setup and Unbounded-Net installation
• add the demo Site, sample application, and connectivity/latency validation script
• update the development tree and machine-client configuration reference
• update E2E prerequisites, commands, variables, layout, and troubleshooting

Draft follow-up

• finish reconciling docs/design.md with the current configuration/authentication model and remove remaining local-file/test-mode wording
• make HTTP connectivity failures fail the demo connectivity validator, not only appear in its summary

Validation

• git diff --check
• bash -n hack/demo/aks-flex-validate-demo-connectivity.sh

This PR contains documentation and demo assets relative to #234.

[Copilot]

Pull request overview

This PR updates AKS Flex Node documentation and demo assets to reflect the controller-backed (ConfigMap + service-proxy) machine endpoint and the no-CNI AKS + Unbounded-Net E2E/cluster networking model, and adds demo manifests/scripts for validating connectivity and drain behavior.

Changes:

• Refresh E2E docs to describe no-CNI AKS provisioning, Unbounded-Net installation, and controller-backed machine goal/repave validation.
• Update configuration and design docs for the in-cluster machine client, ARM-compatible machine response shape (ETag drift key), and status patch model.
• Add demo Site + sample app manifests and a connectivity/latency validation script for demo-site nodes.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
hack/e2e/README.md	Updates E2E flow/prereqs/env vars to include Unbounded-Net + in-cluster controller/registry.
hack/demo/sample-application.yaml	Adds a multi-replica demo workload for drain/placement demonstrations.
hack/demo/demo-site.yaml	Adds a demo Unbounded-Net Site + gateway pool assignment manifest.
hack/demo/aks-flex-validate-demo-connectivity.sh	Adds a demo connectivity/latency validation script for demo-site nodes.
docs/usages/configuration.md	Documents agent.machineClient.* configuration for ARM vs in-cluster mode.
docs/development.md	Updates the project tree to include aks-flex-controller.
docs/design/agent-and-aks.md	Updates design narrative for controller-backed machine goals and ETag-based drift.
docs/design.md	Adjusts high-level design wording for cluster-CNI wiring.

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