[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

[datadog-datadog-prod-us1-2]

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[jhgilbert]

[Automated message] This PR has merge conflicts created by the docs repo reorg project. Because this PR is marked as a work in progress, no attempt was made to auto-resolve the conflicts. When your PR is finished, you can queue your PR for an auto-fix by removing the WORK IN PROGRESS label and the astro-reorg-skip label. Your PR will be processed in the next batch of attempted auto-fixes.

This is an automated comment, but if you have a question, you can mention me in this PR (external contributors) or reach out in #docs-repo-reorg-support on Slack (internal contributors).

[jhgilbert]

[Automated message] This PR has merge conflicts caused by the recent docs repo reorg (files moved from the repo root into hugo/). A new PR with your commits translated to the correct paths has been opened: #39073

Please follow the instructions in the PR description.

This is an automated comment, but if you have a question, you can mention me in this PR (external contributors) or reach out in #docs-repo-reorg-support on Slack (internal contributors).