DOC: Clarify that parquet needs pandas

[pllim]

Description

This pull request is to fix #17972 . No need to run CI but please make sure RTD is green. Thanks!

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[taldcroft]

Thanks @pllim. This also needs to be fixed at https://github.com/d-giles/astropy/blob/8c7593268512594921159000de7a9f885b398fa3/astropy/io/misc/parquet.py#L66-L67.

And in the "nothing is ever easy" category, we probably want to include the standard optional-dependency handling. Somewhere around here we want:

    if not HAS_PANDAS:
        raise ModuleNotFoundError("pandas is required to read parquet files")

[pllim]

Thanks, @taldcroft ! I added a commit to address your review and added you as co-author of that commit.

[taldcroft]

Looks good, thanks!

[pllim]

Hmm. Probably not worth a manual backport. Changing milestone.

p.s. Ugh did GitHub change squash merge logic? Co-author info is lost on squash merge, sorry about that.