DOC: Fix PR02 for pandas.tseries.offsets.CBMonthBegin

[tuhinsharma121]

• related to DOC: Fix docstring validation errors for pandas.core.groupby #60365

Fixes

pandas.tseries.offsets.CBMonthBegin PR02
pandas.tseries.offsets.CBMonthEnd PR02
pandas.tseries.offsets.CustomBusinessMonthBegin PR02
pandas.tseries.offsets.CustomBusinessMonthEnd PR02

Fixed the PR02 error for CBMonthBegin and CBMonthEnd by adding __signature__ attributes to these Cython classes.

Changes Made
File: pandas/_libs/tslibs/offsets.pyx

• Added explicit __signature__ attributes for CBMonthBegin and CBMonthEnd classes
• This allows Python's inspect.signature() to properly introspect these Cython classes
• The signatures match the __init__ method parameters defined in the parent _CustomBusinessMonth class

Why This Fix Works

• Root cause: Cython cdef class definitions don't expose their init signatures to Python's inspection tools
• Solution: Manually set the signature attribute using inspect.Parameter and inspect.Signature
• Result: The numpydoc validator can now verify that docstring parameters match the actual function signature

[sanrishi]

I already working on this @tuhinsharma121
That's why you are getting branch conflicts
See #63866 could you review that pr first ?

[tuhinsharma121]

I forgot to rebase main on this branch, hence the conflict. resolved it. unmerged PRs dont create conflict for other PRs. Nevertheless, happy to close this PR.

[sanrishi]

@tuhinsharma121 It's so helpfull for me if you review my Pr because you are too experienced in it
By the way are you from India?