Add kinematic constraints

[xela-95]

This PR adds the optional feature for specifying kinematic constraints:

• constraints can be specified using SDF frames or URDF massless links;
• multiple constraints can be specified for the same model;
• at the moment, the only contact model supporting them is RelaxedRigidContacts;
• at the moment, there are two types of kinematic constraints, Weld and Connect
  • Weld type constrains all 6 dofs between the two reference frames;
  • Connect type constrains only the 3 positional dofs;

This is not a breaking change since the constraints are optional and can be specified when building the model.

This PR also adds unit tests and documentation.

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📚 Documentation preview 📚: https://jaxsim--399.org.readthedocs.build//399/