feat(aggregation): Make Aggregator and Weighting subclasses public

[ValerianRey]

• Export WeightedAggregator, GramianWeightedAggregator, MatrixWeighting, and GramianWeighting from torchjd.aggregation (added to __all__ and documented in index.rst)
• Update all aggregator and weighting docstrings to reference the most specific public base class (e.g. GramianWeightedAggregator instead of Aggregator, GramianWeighting instead of Weighting)
• Update internal imports that used the private _aggregator_bases path to use the public torchjd.aggregation path
• Make the combine method protected
• Add changelog entry