Move to Docling's Hybrid Chunking

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We're using a legacy docling version, custom parsing of the docling v1 json, and our own chunking. Docling has a hybrid chunker that was introduced since our initial Docling implementation that should remove the need for us to do our own parsing of the Docling json or our own chunking.

The RAG work in instructlab/instructlab already uses the HybridChunker, so this is scoped to just getting SDG on a current version of Docling and the same chunking strategy.

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This is related but not identical to #374 , as that one encompasses moving to a new version of Docling but this is about moving to HybridChunker specifically.