docs: add CONTRIBUTING and CODE_OF_CONDUCT

[dafahaha]

Summary

This PR adds standard community files to help new contributors understand how to participate and establish community norms.

Changes

1. CONTRIBUTING.md: Contribution guidelines, code style (PEP 8), testing requirements, PR process, and issue reporting template
2. CODE_OF_CONDUCT.md: Contributor Covenant v2.0, providing a clear code of conduct for the community

Motivation

These standard files are essential for:

• Helping new contributors get started quickly
• Establishing community norms and expectations
• Making the project more welcoming and professional

Testing

• All files are standard templates commonly used in open-source projects
• No functional code changes, only documentation