[codex] Add contributing entrypoint

[wenlei0603]

Summary

• add a root CONTRIBUTING.md entrypoint for GitHub's contributing-guidelines link
• point contributors to the rendered Read the Docs guide and the source docs/source/contributing.rst

Context

This addresses the root CONTRIBUTING.md suggestion from #834 without duplicating the full contributing guide in two places.

Validation

• Confirmed docs/source/contributing.rst exists in the repository
• Confirmed https://quanteconpy.readthedocs.io/en/latest/contributing.html returns HTTP 200