[Data][Docs] Remove "Data Representations (internal)"

[bveeramani]

Why are these changes needed?

We shouldn't document Data internals that aren't relevant to developers.

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[amogkam]

seems like block is being referenced here /ray/doc/source/data/examples/custom-datasource.rst:97: WARNING: undefined label: block-api, and generally is part of the DeveloperAPI for creating new datasources.

[bveeramani]

seems like block is being referenced here /ray/doc/source/data/examples/custom-datasource.rst:97: WARNING: undefined label: block-api, and generally is part of the DeveloperAPI for creating new datasources.

@amogkam where would we put the Block reference? Maybe somewhere in the Dataset reference?

[amogkam]

I think it's fine to keep under Data Representations. And we can downgrade it to developer API.