Corrected the fr3v2 colors and added models for fr3_duo and mob…

[AndreasKuhner]

Description

3 Tasks Done

1. Corrected color scheme of fr3_v2
2. Added franka_fr3_duo
3. Added franka_mobile_fr3_duo_v0_2

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): Already within
• 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)