model-zoo tutorial

[Can-Zhao]

Is your feature request related to a problem? Please describe.
No example for users to use models from model-zoo

Describe the solution you'd like
Add two examples:

1. use model from model-zoo directly, apply to a different dataset (readme doc)
2. use model from model-zoo for transfer learning, apply to a different task (pytorch code)

Describe alternatives you've considered

Additional context
Ref:
https://github.com/Project-MONAI/model-zoo/releases/tag/hosting_storage_v1
https://github.com/Project-MONAI/MONAI/blob/dev/monai/bundle/scripts.py#L230
#873
Project-MONAI/MONAI#4994