Add CONTRIBUTING.md to guide contributors and copilot

[Copilot]

This PR adds a comprehensive CONTRIBUTING.md file to the repository root to standardize and streamline contributions from the community and coding copilots.

What's Added

The CONTRIBUTING.md file includes the following sections:

• Getting Started: Prerequisites (Node.js v18.16.1+, Rust v1.70.0+), platform-specific tools, and initial setup instructions
• Development Workflow: Step-by-step guide for forking, cloning, branching, and creating pull requests
• Coding Standards: References to existing style guides for all supported languages:
  • TypeScript (Google TypeScript Style Guide)
  • Rust (Rustfmt Style Guide)
  • C/C++ (Google C++ Style Guide)
  • C# (Microsoft C# Style Guide)
  • ShaderLab (Unity ShaderLab Style Guide)
• Building and Testing: Platform-specific build commands (make darwin, make android, make windows) with available options
• Commit Message Conventions: Conventional commit format with examples
• Pull Request Process: Guidelines for submitting and reviewing PRs
• Code of Conduct: Community guidelines for respectful collaboration
• License: Apache 2.0 license reference
• Additional Resources: Links to documentation, API references, and help resources

Benefits

This will help:

• New contributors get started quickly with clear setup instructions
• Maintain consistent code quality across the multi-language codebase
• Standardize the contribution workflow and PR process
• Improve collaboration between human contributors and AI coding assistants
• Reduce maintainer overhead by providing self-service documentation

The file references existing documentation in docs/contributing/ and follows GitHub's best practices for CONTRIBUTING.md files.

Fixes #137.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• registry.npmmirror.com
  • Triggering command: npm install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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[yorkie]

Please add a note to the related coding style: using 2 spaces as the default source indents.

[Copilot]

Please add a note to the related coding style: using 2 spaces as the default source indents.

Added explicit note about 2-space indentation standard to the Code Formatting section. The note references how this is enforced by the automated formatting tools and configured in the project's formatting configuration files (e0d209b).