Document Terraform v5 migration guidance

[shibayan]

Summary

• clarify that the in-place v4-to-v5 migration does not apply to the former Terraform module
• recommend deploying a new Flex Consumption environment with the current v5 module
• document how to reuse an existing Key Vault and which access settings must be verified

Why

The v4 and v5 Terraform modules have substantially different resource structures, so a Terraform state/import-based in-place migration is not supported. The migration guide should make the recommended re-setup path explicit.

Validation

• npm run build in docs

Closes polymind-inc/terraform-azurerm-acmebot#35

[Copilot]

Pull request overview

Updates the v4→v5 migration documentation to explicitly address Terraform-managed deployments, clarifying that the “in-place migration” guidance applies to an existing Function App deployment, not to the legacy Terraform module deployment model.

Changes:

• Adds a “Terraform Module Deployments” section clarifying the legacy shibayan/keyvault-acmebot/azurerm module is not eligible for the in-place migration steps.
• Documents the recommended approach for Terraform users: deploy a fresh v5 Flex Consumption environment using the polymind-inc/acmebot/azurerm module and retire the v4 environment.
• Notes Key Vault reuse via acmebot.vault_uri and calls out the configuration areas that must be recreated/verified for the new deployment.

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