DOC - improve documentation for Drop #1756

[wikip50700]

This PR improves the documentation for the Drop class by adding docstrings and a user guide section with an example showing how to use Drop with ApplyToCols.

[rcap107]

Suggested change

	column-wise contexts such as :class:`ApplyToCols`, data ops, or internally
	column-wise contexts such as :class:`ApplyToCols`, Data Ops, or internally

[rcap107]

Hi @wikip50700, thanks for this PR. I think it's already in a good shape, however before merging I would like to add also an example with the Data Ops similar to what is written in the original issue #1756:

.skb.apply(skrub.choose_from([StringEncoder(), Drop()]))

The reasoning for the choose_from there is that it's not clear whether the given column is useful or not, so it would be good to find that out using hyperparmeter tuning. The example should be in /doc/modules/data_ops/validation/hyperparameter_tuning.rst, after the section on feature selection.