[DOCS-15112] Document Azure Secretless Auth setup and migration

[buraizu]

What does this PR do? What is the motivation?

Fixes DOCS-15112

Documents Azure Secretless Auth (currently in Preview) on the Getting Started with Azure page and cross-links the migration flow from two Azure guides:

• Getting Started with Azure — adds an "Authentication methods" section comparing Secretless Auth and Client Secret, adds the Authentication Method selection step to the Quickstart and Terraform flows, adds a collapsible "Migrate an existing app registration to Secretless Auth" section with per-method tabs (Azure CLI, Terraform, Azure Portal), and updates the Terraform "Use an existing app registration" example to use secretless_auth_enabled.
• Azure Resource Manager (ARM) guide — adds a Next steps link to the migration flow.
• Azure advanced configuration guide — notes the federated-credential model alongside the client-secret architecture and links to the migration flow.

Merge instructions

Draft / work in progress. Not ready for review or merge yet — see Additional notes.

Merge readiness:

• Ready for merge

Additional notes

Still to confirm before this is review-ready:

• Site region restrictions (pending confirmation).

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/docs15112/azure-secretless-auth/getting_started/integrations/azure
• https://docs-staging.datadoghq.com/docs15112/azure-secretless-auth/integrations/guide/azure-advanced-configuration
• https://docs-staging.datadoghq.com/docs15112/azure-secretless-auth/integrations/guide/azure-resource-manager