[AGNTLOG-229 Multiline features FAQ

[DDuongNguyen]

What does this PR do? What is the motivation?

This PR adds a user-friendly FAQ section to the multi-line log aggregation documentation to help users better understand when and how to use different multi-line aggregation approaches.

The motivation is to reduce confusion and help users choose the right multi-line aggregation approach for their specific needs.

Merge instructions

Merge readiness:

• [] Ready for merge

Additional notes

This is a documentation-only change that improves clarity without modifying any functionality.

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/yoon/multiline-faq/agent/logs/advanced_log_collection

[DDuongNguyen]

/merge

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