[RUM-8832] Clarify SDK and RUM initialization timing requirements in setup.md

[ncreated]

What does this PR do? What is the motivation?

Updated the public documentation to clearly state that the Datadog SDK and RUM should be initialized in AppDelegate to ensure accurate telemetry (e.g., application startup duration).

Also added guidance for SwiftUI apps using @UIApplicationDelegateAdaptor.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

This can be merged without waiting for upcoming 3.0.0 release.

[github-actions]

📝 Documentation Team Review Required

This pull request requires approval from the @DataDog/documentation team before it can be merged.

Please ensure your changes follow our documentation guidelines and wait for a team member to review and approve your changes.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/ncreated/update-sdk-and-rum-init/real_user_monitoring/mobile_and_tv_monitoring/ios/setup