How to treat the fixed joint

Hello!
This issue is for asking/discussing with you how it could be the best way to handle the fixed joints in mujoco.

Our URDFs (e.g., ergoCub, iCub) have plenty of fixed joints, for example, for defining the FT sensors, additional frames, but from my understanding, once converted to mujoco xml the links attached to fixed joints are merged into one, removing the fixed joints.

My question is, how do you handle them? How do you define the FT sensors without those joints?
I saw that you handled the sensor frames here:
- https://github.com/gbionics/mujoco-urdf-loader/issues/6#issuecomment-2233581355

I saw that there is [`equality/weld`](https://mujoco.readthedocs.io/en/stable/XMLreference.html#equality-weld), maybe it could be interesting for preserving the fixed joints

cc @traversaro @Giulero @giotherobot @sksubiit @martinaxgloria @AntonioViscomi