[DOC] Add usage examples to dataset deletion functions - #1707

In get_datasets the docstring section header is misindented: Examples is currently aligned under the Returns item (extra leading spaces) while the underline -------- is at the normal indentation. This breaks NumPy/numpydoc formatting and can render incorrectly in generated docs. Align Examples with other section headers (same indentation as Returns) and add the usual blank line separation.

The _topic_delete_dataset docstring example uses openml.datasets.functions._topic_delete_dataset(...) after only import openml. openml.datasets does not expose a functions attribute via its __init__.py, so this example will raise AttributeError unless the submodule is explicitly imported. Adjust the example to import the function/submodule explicitly or show an access path that works from import openml alone.

There are two blank lines between the last example statement and the closing triple-quote, which is inconsistent with the surrounding docstrings and can lead to odd spacing in rendered docs. Remove the extra blank line so the docstring ends right after a single blank line following the examples.

PR description/title say this change adds usage examples to delete_dataset and _topic_delete_dataset, but this diff also adds examples to list_qualities, list_datasets, and get_datasets. Please either update the PR description to match the actual scope, or split the additional doc changes into a separate PR to keep review scope focused.