How can I create a new environment in the MuJoCo playground?

Hi all,
I would like to add my own robot arm model and define custom task objectives in the MuJoCo playground. However, I couldn’t find a clear entry point in the repo for modifying or extending the environment.

Could you please advise on how I should proceed if I want to:

1.Integrate a new robot arm model.
2.Define a new task or objective for the environment.

Is there any recommended way or existing extension interface to achieve this?

Thank you!