DOC: Add docstrings for MultiIndex.levels and MultiIndex.codes

[datapythonista]

xref #55148

Seems like those docstrings are empty, we should create them.

See the attributes section here: https://pandas.pydata.org/docs/reference/api/pandas.MultiIndex.html

The docstring for `MultiIndex.levels should include information to make clear that levels are preserved even if the dataframe using the index doesn't contain all levels. See this page in the docs: https://pandas.pydata.org/docs/user_guide/advanced.html#defined-levels and this comment: #55433 (review)

[shiersansi]

This is the second time I've brought up pr in an open source project, so I misunderstood what you meant, and I'll finish the issue again.

[datapythonista]

It's normal, and the issue was difficult to follow, since it was a discussion, but I think the new issue explains better what needs to be done. If you have any question or you need help we are here to help. Thank you!

[datapythonista]

We've got now the docstring for MultiIndex.levels, but the one for MultiIndex.codes is still missing. Labelling this as good first issue in case anyone wants to help.

[mileslow]

take

[AdventurousDataScientist]

Hi mileslow do you still need time for this task, or do you mind if I work on it?

[mileslow]

@Rollingterminator1 go for it.