More cross-references in the expressions documentation

[jeromedockes]

Adding more links and "See also" sections and references to the examples in the docstrings of objects in the expressions module

[MarieSacksick]

Exemples of cross-references to add:

• [x] in cross_validate,
• [x] in describe_steps, add "See Also" for draw_graph and full_report.
• [x] in drop add a link to selectors
• [x] in select add a link to selectors
• [x] in get_grid_search add the link towards sklearn GridSearchCV (as it's done for RandomizedSearchCV in get_randomized_search)
• [x] in train_test_split, in splitter, add redirect to documentation in scikit-learn.
• [x] in is_y add link to the function mark_as_y
• [x] in is_X, add link to the function mark_as_X
• [x] in mark_as_y add See also to y directly.
• [x] in mark_as_x add See also to X directly.

PS : could this issue be added to the project https://github.com/orgs/skrub-data/projects/6 please?

[jeromedockes]

thanks @MarieSacksick !!

[MarieSacksick]

I can take this one.