add example dags

from airflow import DAG

from airflow.operators.bash import BashOperator

from datetime import datetime, timedelta

default_args = {

    "owner": "airflow",

    "depends_on_past": False,

    "email": ["bozhang@nao.cas.cn"],

    "email_on_failure": False,

    "email_on_retry": False,

    "retries": 1,

    "retry_delay": timedelta(minutes=5),

    "params": {"example_param": "example_value", "a": 1, "b": 2, "c": 3},

}

with DAG(

    dag_id="dag_hello_world",

    default_args=default_args,

    description="A simple tutorial DAG",

    start_date=datetime(2023, 1, 1),

    schedule="@daily",

    catchup=False,

    tags={"example"},

) as dag:

    t1 = BashOperator(

        task_id="print_date",

        bash_command="date",

    )

    t2 = BashOperator(

        task_id="sleep",

        bash_command="sleep 5",

        retries=3,

    )

    t1 >> t2