Add "Next Steps" doc/epilog at the end of running init and build

[kaxil]

Similar to the following, there should be a "Next Steps" doc printed after someone runs airflowctl build to show them the following and their usage:

For init:

• settings.yaml (airflow_version and python_version)
• requirements.txt
• dags directory

For build:

• Running DAGs
• Activating venv and running airflow commands
• Changing executor (and configuring postgres) (probably ??)

https://github.com/kaxil/airflowctl/blob/e4f8fe129a02572b9ad2e2e74b57e14e07324c35/airflowctl/cli.py#L431-L441