Add the ROBOTIS OMY-3M to menagerie.

[kimtaehyeong99]

Description

Add the ROBOTIS OMY-3M to menagerie.

Fixes:

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)
• Updated CHANGELOG.md:
  • Global changelog (if your change affects the overall repo)
  • 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.

[google-cla]

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