academy: Dutch Haven tutorial with kind-based oefencluster

[MWest2020]

Summary

• New Dutch academy module that explains the VNG Haven standard end-to-end and dispels the common "Haven vs Haven+" confusion (one standard; "Haven+" is a category inside the Compliancy Checker, not a separate tier).
• Pairs with a kind-based oefencluster using Gateway API + Envoy Gateway, metrics-server, and a verified run of the Haven Compliancy Checker (v12.8.0) — the actual 7/15 results table from a 4-node kind cluster is included as concrete reference.
• Content grounded in canonical sources: checks.yaml in the Haven GitLab repo and the haven.commonground.nl techniek docs. Provider list pulled from the official Aan de slag page.

Refs ConductionNL/.github#49

Test plan

• Read the module for tone and accuracy, especially section 3 (Haven+) — it overrides the framing in the original issue.
• Spin up the 4-node kind cluster from section 5 on real Docker (rootless Podman blocks this; rootful Podman or Docker works).
• Run through Envoy Gateway + metrics-server install and confirm the article's flow is correct (the kubectl apply gateway-api/standard-install.yaml step is intentionally absent — the Helm chart bundles the CRDs).
• Verify port-forward on 18080 + curl returns the nginx welcome page.
• Run haven check (v12.8.0+) against the cluster and confirm the table in section 6 still matches (counts may shift with new Haven releases).
• Linkcheck on external URLs to Haven, VNG, developer.overheid.nl, Bright Cubes, Envoy Gateway, kind, etc.

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