[DOCS-14474] Clarify PAYG account support in Azure CCM setup

[domalessi]

What does this PR do? What is the motivation?

Fixes DOCS-14474

Clarifies Azure Cloud Cost Management's support for Pay as you go (PAYG) accounts. The previous Overview said only that PAYG support was in Preview and to contact support with setup issues, without naming the actual eligibility requirement or workarounds.

Changes:

• Promotes PAYG account support to its own top-level section between Overview and Setup, so eligibility is visible before users start setup.
• States the specific requirement: Cloud Cost Management needs actual and amortized cost exports.
• Names the failure mode: PAYG subscriptions limited to Usage details (usage only) exports cannot send cost data to Datadog.
• Links Microsoft's Cost Management export support matrix so users can check their subscription.
• Adds a migration path (MCA or EA) for PAYG subscriptions that lack the required exports.
• Updates the Overview to acknowledge EA accounts alongside MCA (both are valid at all three scopes).

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/domalessi/docs-14474-azure-ccm-payg/cloud_cost_management/setup/azure