[Python] Use docstring-adder to add runtime docstrings to type stubs #55596

Replace python/scripts/update_stub_docstrings.py (libcst codemod) with docstring-adder for inserting runtime docstrings into the type stubs at wheel-build time.

• Small wrapper: stage an importable pyarrow, run docstring-adder, then ruff format.
• Build Cython with embedsignature.format=clinic so __doc__ carries no signature line.
• Drop libcst from the build requirements; docstring-adder (git-only) goes in requirements-wheel-build.txt.
• docstring-adder skips name = _clone_signature(other) assignments in compute.pyi; needs an upstream change or a small post-pass.