Improve utilities provided by JaxsimDataclass

[diegoferigo]

When I first designed this class, I wanted to keep the Mutability enum private. However, in practice, there are multiple use-cases in which downstream projects need to use it.

This PR:

• Adjusts methods to expose publicly the Mutability enum.
• Reduces the number of copies, possibly improving the memory footprint of downstream code.
• Uses stricter checks to prevent JIT recompilations. Beyond pytree structure, now also the shape and dtype of the leaves if checked, if they are arrays.
• Adds docstrings.

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📚 Documentation preview 📚: https://jaxsim--116.org.readthedocs.build//116/

[diegoferigo]

Moving this forward since it's not merging into main. @flferretti if you have comments, please leave them in #108.