Add comprehensive contributor documentation

[jamiechicago312]

Summary

This PR adds comprehensive contributor documentation to help new and existing contributors understand how to get involved with the OpenHands project.

Changes Made

📚 New Documentation

• Added openhands/usage/contributing.mdx - A comprehensive contributor guide that covers:
  • Project vision and values ("Free as in Freedom")
  • Different ways to contribute (Frontend, Backend, Agent Development, Testing, Documentation, Research)
  • Getting started guide with development environment setup
  • Pull request process and guidelines
  • Community guidelines and code of conduct
  • Licensing information

🔧 Configuration Updates

• Updated docs.json to include the new contributing page in the navigation under "Developers" section
• Updated Slack link from https://dub.sh/openhands to https://openhands.dev/joinslack for consistency

Why This Matters

This documentation addresses a critical need for the OpenHands project:

• Lowers the barrier to entry for new contributors
• Provides clear guidance on different ways to contribute
• Establishes community standards and expectations
• Aligns with the project's open-source values and mission

Content Highlights

The contributor guide includes:

• 🌟 Clear articulation of the project's vision
• 🎯 Specific contribution areas with examples
• 🚀 Step-by-step getting started instructions
• 📋 Pull request guidelines and best practices
• 🤝 Community guidelines and code of conduct
• 📄 Clear licensing information

Testing

• ✅ Verified the new page renders correctly in the documentation structure
• ✅ Confirmed all internal and external links are functional
• ✅ Validated the navigation structure includes the new page appropriately

Related Issues

Fixes #1

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This PR represents a significant step toward making OpenHands more accessible to contributors and building a stronger, more inclusive community around the project.

@jamiechicago312 can click here to continue refining the PR

[enyst]

Thank you! It gets a bit messy with these transitions 😅