Run GPU jobs from Airflow with Modal

Modal can now be triggered directly from an Airflow DAG, letting teams run GPU-backed AI/ML tasks without setting up Celery or Kubernetes executors. The guide covers installing Modal in an Airflow environment, setting MODAL_TOKEN_ID and MODAL_TOKEN_SECRET, and two approaches: calling deployed Modal Functions via lookup, or building a custom operator that runs Python in Modal Sandboxes. A Modal Airflow provider package is in progress.