[DOC] Improved docstring offset class BusinessDay

[SebastianGoldmann]

• xref DOC: Enforce Numpy Docstring Validation (Parent Issue) #58063 DOC: Enforce Numpy Docstring Validation
• 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.

First contribution, looking forward to your feedback.

[SebastianGoldmann]

I tried resolving the PR02 error on the offset businessday cython class in 2 ways:

1. Add the init in the stub file of offset.pyi. => failed, python does not recognize this
2. add the init to the pyx file => failed, python still does not show the init and still returns the args kwargs init. Actual init is defined in the classes it inherits from.

I conclude that the PR02 cannot be resolved for cython classes as it would require additional unnecesarry code.

Please let me know if you think otherwise.

kr,
Sebastian