Add docs around when to use TaskGroup vs SubDag and potentially listing PROs and CONS.

[kaxil]

It would be great for users to know when they should use TaskGroup vs SubDag. A section somewhere in docs would be great or even better a Matrix / table to explain difference would be aweomse.

What are the PROs and CONs of each

TaskGroup: https://airflow.readthedocs.io/en/latest/concepts.html#taskgroup
SubDags: https://airflow.readthedocs.io/en/latest/concepts.html#subdags

[VeenaArv]

Hi, can I work on this as my first issue? Should this go under concepts or somewhere else?

[kaxil]

@VeenaArv Yes somewhere in Concepts, in or after https://github.com/apache/airflow/blob/master/docs/concepts.rst#taskgroup section

[turbaszek]

@kaxil @VeenaArv should we also add information about TriggerDagRunOperator as this is also a possibility to manage large DAGs?