DOC: known issue about pandas Series multiplication

[MridulS]

Description

This PR adds a section to https://docs.astropy.org/en/latest/known_issues.html regarding the fragile behavior with pd.Series objects.

Fixes #11247

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[MridulS]

Hmm, this is failing as pandas isn't installed in the minimal dependencies environment and in the olddeps environment it fails with pandas 1.4.

[mhvk]

On the test failure, there is

.. doctest-requires:: pandas

but I'm not sure we can give this a minimum version.

To be honest, I'm fine with just skipping the tests...

[mhvk]

Thanks, @MridulS! Looks all good to me, but I'll let @eerovaher have a last look too.

[eerovaher]

This is almost ready, but there's a trailing whitespace somewhere that is causing the pre-commit.ci job to fail.

[pllim]

pre-commit.ci autofix

[pllim]

@eerovaher , the bot took care of the trailing whitespace. Does this look good to you now? We can always use the squash and merge button for this PR.

Thanks, all!