docs: Add CONTRIBUTION_PLAN.md for GitHub use case
This Pull Request adds a detailed CONTRIBUTION_PLAN.md that documents the process of evaluating and contributing to the pandas-dev/pandas project. The plan includes environment setup, issue identification, and a step-by-step guide to raising a PR.

This is part of a simulated GitHub use case to demonstrate contribution readiness.
This is already covered in our documentation, https://pandas.pydata.org/docs/development/index.html. Closing