refactor(linalg): Make Matrix and PSDMatrix public

[ValerianRey]

Summary

• Introduces a new public torchjd.linalg package exposing Matrix and PSDMatrix (the rest of _linalg stays protected)
• Makes MatrixWeighting and GramianWeighting protected. These classes are still used to specify the docstring of the __call__ methods of the aggregators, but the user only sees those aggregators as Weighting[Matrix] and Weighting[PSDMatrix], respectively. The MatrixWeighting and GramianWeighting classes really just bring updated docstrings, that's all.
• Makes the public type of the gramian_weighting of GramianWeightedAggregator be Weighting[PSDMatrix] instead of GramianWeighting, so that feat(aggregation): Add CRMOGMWeighting #669 can work. Similar with weighting of WeightedAggregator being Weighting[Matrix].
• Expands docstrings on Matrix and PSDMatrix with Jacobian and Gramian examples; adds Sphinx documentation under a new linalg section in the API Reference

[ValerianRey]

We either need to have both _linalg (protected) and linalg (public), or we could have _linalg and types, or _linalg and export those types to the torchjd package itself. @PierreQuinton I'd love your opinion on that.