Add a short TableGen HowTo to tutorial chapter 2.

Add a note to chapter 2 of the Toy tutorial stating how to invoke
mlir-tblgen to check the generated C++ code. IMHO this is incredibly
useful when getting acquainted with TableGen/ODS.

Closes tensorflow/mlir#228

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/228 from ombre5733:toy-ch2-howto-mlir-tblgen a051a3734ca8bbf4f12027fe737aca07c64ca59d
PiperOrigin-RevId: 279518989