[DCV-3894] Add Azure Key Vault secrets backend how-to docs

[gams87]

Adds documentation for using Azure Key Vault as an additional (secondary) Airflow secrets backend, validated end-to-end on Airflow 2 and Airflow 3 environments.

New pages

• How-tos > Datacoves > Configure Projects > Configure Azure Key Vault (how_to_configure_azure_key_vault.md): admin setup guide covering both authentication methods (Managed Identity on Azure-hosted Datacoves with a vault-URL-only config, and service principal with a client secret for any deployment), secret naming rules (airflow-variables- prefix, required datacoves- key prefix, underscore-to-dash translation), and project-level vs environment-level configuration. Includes troubleshooting notes hit during real testing: RBAC data plane roles vs management roles, Key Vault Secrets Officer vs Key Vault Secrets User, the easily-confused Certificates roles, and role assignment propagation delays.
• How-tos > Airflow > Secrets - Azure Key Vault (use-azure-key-vault.mdx): DAG usage guide with the variable lookup flow, naming table, best practices, an example DAG (portable Airflow 2/3 imports, tested as-is), and how to audit secret fetches with Key Vault diagnostic logging.

Changed

• use-aws-secrets-manager.mdx: intro line now mentions Azure Key Vault as a supported backend.

Notes

• The workbench "Additional Secrets Backend" dropdown gains an Azure Key Vault option in the corresponding datacoves repo branch (DCV-3894-airflow-azure-keyvault-secrets), and its help text links to the new configure page, so this PR should be published before or together with that release.

🤖 Generated with Claude Code

[noel]

@gams87 we should show how to use it without Datacoves secrets manager, an example like this
https://docs.datacoves.com/docs/how-tos/airflow/use-aws-secrets-manager#configure-an-airflow-connection

[noel]

see comment

[gams87]

@noel done.