Rainbow robotics rby1 _ new model update

[dongridong]

Description

This PR adds MuJoCo files for Rainbow Robotics’ dual-arm manipulators, RBY1A and RBY1M (v1.2 and v1.3).
To make it easier for users to attach their own custom grippers, I also added a no_gripper model.

Please also note that I had to open a new PR because the previous PR was not merged successfully.
For reference, the previous PR is here: #150

I would appreciate it if you could review this PR, and please let me know if there are any changes that need to be made.

Have a great day :)

Fixes: Removed the legacy models and added new models (RBY1A, RBY1M — v1.2, v1.3, and no_gripper).
#150

Checklist

Please check off each item ([x]) once complete, or mark it as [N/A] if it doesn't apply:

• Added your name to CONTRIBUTORS.md (alphabetically by first name)
• [N/A] Updated CHANGELOG.md:
  • [N/A] Global changelog (if your change affects the overall repo)
  • [N/A] Model-specific changelog (if it affects a specific model only)
• Followed the XML formatting/style guidelines (if editing MJCF)
• Ran pytest test/ locally and ensured all tests pass
• Signed the Contributor License Agreement (CLA)

Refer to the contributing guide if you're unsure about any of the steps.