Add Dialect in op definition to capture prefix and documentation.

    Enables specifying the documentation for dialect along with defining the ops of the dialect. The doc generator will be expanded in follow up to emit the documentation in the autogenerated files. This is precursor to allowing common base for all ops in a dialect.

    All the dialect documentation is super sparse and just added as placeholder.

    I was tempted (and started) to move ConstantOp to be generated too, but this will be easier post adding extra_methods, so deferring until then.

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PiperOrigin-RevId: 245759984