[tmva] Unify `RBatchGenerator_TensorFlow` and `TMVA_Higgs_Classification` examples

[guitargeek]

The RBatchGenerator_TensorFlow.py tutorial is very bare bones right now. It would be better if it implements a meaningful example, like the Higgs classification with Keras in TMVA_Higgs_Classification.C.

The resulting trained models should be similar, so we can showcase that the new RBatchGenerator can provide the same functionality to the user that was formerly accessible through PyMVA: training Keras models on ROOT data.

Then, we can also use the resulting model from the RBatchGenerator tutorial in the other tutorials that showcase inference with SOFIE, without depending on the PyMVA tutorials (which are disabled because they don't support Keras 3 anymore).