feat: refactor stackit edge cloud provisioning

[Matthiator]

📝 Summary

Refactors STACKIT Edge Cloud provisioning so Edge Cloud can be handled as a separate, opinionated provisioning path next to SKE.

This PR:

• Adds separate Terraform modules for Edge Cloud instance creation, Edge image upload, and Edge host provisioning.
• Reworks the generated STACKIT Terraform example for the new Edge Cloud flow.
• Splits STACKIT provider docs into Terraform bootstrap, SKE provisioning, and Edge Cloud provisioning pages.
• Documents the current single-node Edge Cloud ingress approach with STACKIT public IP, private VM IP, MetalLB, and external-dns.
• Updates generated Helm/Terraform templates and tests for the new Edge Cloud shape.
• Fixes Longhorn dashboard exposure below the /longhorn Traefik subpath by rendering the required redirect and path rewrite middlewares.

🧩 Type of change

• 🔧 CLI / Go code
• 📦 Helm chart
• 🧱 Terraform module
• 📝 Documentation
• 🧪 Test or CI change
• ♻️ Refactor / cleanup

⚠️ Is this a breaking change?

• Yes, this change breaks existing functionality (explain in summary)

The previous Edge Cloud Terraform input/output shape is replaced. Legacy single-host Edge inputs are intentionally removed in favor of the new Edge instance, image, and hosts structure.

🧪 Testing

• CI passed
• Manually tested (local/dev cluster)
• Unit tested
• Not tested (explain why below)

Manual validation so far:

• Built the kubara binary locally and generated Terraform/Helm artifacts for an Edge Cloud test repository.
• Applied the Edge Cloud Terraform flow far enough to provision the Edge instance, uploaded image, and VM host.
• Bootstrapped Argo CD and deployed the managed Helm stack on the Edge Cloud cluster.
• Validated Longhorn dashboard routing behind the Traefik /longhorn subpath.
• Ran make docs-build successfully.
• Ran make test successfully.

Still pending before marking this PR ready:

• Full CI result.

🔗 Related Issues / Tickets

✅ Checklist

• Code compiles and passes all tests
• Linting and style checks pass
• Comments added for complex logic
• Documentation updated (if applicable)

📎 Additional Context (optional)

This PR is intentionally opened as draft while the Edge Cloud flow is still being validated end-to-end.

[fabian-schmitt]

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