[FEATURE] Add Unitree G1 Walking Demo

[0nhc]

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I think it's cool to have another humanoid RL demo, so I added an RL demo for Unitree G1 walking in this PR. Here's my modification:

• Added Unitree G1's urdf under assets/urdf, with original BSD-3 License.

• Added 3 Python scripts (g1_env.py, g1_train.py, g1_eval.py) under examples/locomotion.

Demo video after training for 69.5 seconds on my PC:

https://github.com/user-attachments/assets/f8c37f61-1101-407e-81ac-a9b53c969927