The `train_test_split_dataset` Helper Function for Multi-table Splits is not Officially Documented

[lucaurelien]

The train_test_split_dataset function is not part of the official Khiops Python API or documentation, but it appears in some example notebooks to help split multi-table datasets consistently.

[folmos-at-orange]

Fix: add helpers to the autosummary section in ./doc/sklearn/index.rst

[popescu-v]

Closed by PR #425 .