Feature request: Native support for tracked robot simulation in MuJoCo

### The feature, motivation and pitch

Hi MuJoCo team,

I would like to request native support for tracked / crawler vehicle simulation.

Tracked robots are widely used in robotics for inspection, rescue, rough-terrain mobility, and mobile manipulation. They are also important in reinforcement learning and control research. However, MuJoCo currently seems to lack a standard and robust way to model tracked vehicles.

Current workarounds are not ideal:
- differential-wheel approximations are too crude,
- explicit track-link chains are complex and expensive,
- custom contact hacks are difficult to generalize.

As a result, MuJoCo is currently less convenient for an important class of mobile robots. A native tracked-vehicle abstraction, or even a simplified but stable official solution for track-ground interaction, would greatly improve its usefulness for robotics.

This would be especially valuable for stair climbing, slope traversal, obstacle crossing, rough-terrain locomotion, and RL for tracked robots.

Thanks for considering this feature request.

### Alternatives

_No response_

### Additional context

_No response_