docs: add comprehensive Apache Airflow tutorial for backend onboarding

[nitaibezerra]

Summary

• Add comprehensive Apache Airflow tutorial (docs/onboarding/airflow-tutorial.md) covering:

  • Introduction and motivation (why Airflow over alternatives)
  • Architecture and infrastructure components (Scheduler, Web UI, Triggerer, DB, Workers)
  • Local setup with Astro CLI
  • Fundamental concepts (DAG, tasks, operators, dependencies)
  • Essential operators (PythonOperator, BashOperator, Sensors)
  • Intermediate concepts (execution_date, backfill, XComs, Variables, Connections, TaskFlow API)
  • Advanced patterns (branching, dynamic task mapping, task groups, trigger rules)
  • Best practices and anti-patterns
  • Monitoring and debugging
  • Application to DestaquesGovBr pipeline with realistic DAG example
  • 5 practical exercises (beginner to advanced)
  • Complete glossary of terms

• Update mkdocs.yml with navigation entry

• Update roteiro-onboarding.md:

  • Add Airflow to backend developer track
  • Update mermaid diagram
  • Add Airflow exercises
  • Add Airflow to competencies table
  • Add Airflow items to backend checklist

Test plan

• Review tutorial content for accuracy
• Verify mermaid diagrams render correctly
• Test code examples locally with Astro CLI
• Build docs with mkdocs build to verify no broken links
• Navigate through the tutorial in rendered documentation

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