BUG: Add unordered option to pandas.cut (#33141)

[mabelvj]

Added tests and error message when there are no labels and ordered=False.

Issue: Pandas cut raises error if labels are non-unique (Closes #33141)

• closes Pandas cut method gives an error if labels are non-unique #33141
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[mabelvj]

• Updated the documentation in pandas.cut.
• Added an example.
• Run the documentation validation script.
• Also added a new line in bugs -> reshaping. Should a new line about the new functionality be added in the Enhancements section?

[jschendel]

Thanks! A few comments.

[jschendel]

lgtm, I think merging master will fix the CI failure

[jschendel]

cc @pandas-dev/pandas-core

[mabelvj]

Updated:
labels = Categorical(
labels,
categories=labels if len(set(labels)) == len(labels) else None,
ordered=ordered,
)

[mabelvj]

Updated:

            labels = Categorical(
                labels,
                categories=labels if len(set(labels)) == len(labels) else None,
                ordered=ordered,
            )

instead of:

            labels = Categorical(
                labels,
                categories=labels if ordered else None,
                ordered=ordered,
            )

since we need to also handle the case of unordered labels. We just need to avoid passing duplicates labels to categories and also raise an error when we are using ordered=True and there are duplicate labels.

[jreback]

thanks @mabelvj very nice!