Document some more categorical methods

[ivirshup]

• closes DOC: Document a few more methods for Categorical Array #56927
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Opening as a draft to make sure it renders

[ivirshup]

I've added references to these methods in doc/source/reference/arrays.rst so that pages get generated for the methods. Tbh I didn't know that there was a separate location for documenting the class here.

I don't know how much needs writing about the methods here since they all have docstrings.

[mroeschke]

Thanks @ivirshup