docs+feat: improve contributing guide and add clean_examples utility

[devin-ai-integration[bot]]

Improve Contributing Guide and Add Development Utility

This PR improves the CONTRIBUTING.md guide with clearer instructions and adds a new utility script to help developers maintain a clean development environment.

Changes

• Add more detailed instructions for development workflow
• Clarify type stub generation process and requirements
• Add comprehensive troubleshooting section
• Add clean_examples.py utility script for managing example apps
• Improve formatting and organization of the guide

Testing

• ✅ Verified all documented processes work as described
• ✅ Tested clean_examples.py script functionality
• ✅ All pre-commit checks pass
• ✅ No changes to generated .pyi files needed

Additional Notes

• The clean_examples.py script helps developers maintain a clean environment when switching between different example apps
• Added clarification about development server behavior and frontend-only mode
• Improved organization of pre-commit and testing instructions

Link to Devin run: https://app.devin.ai/sessions/66907725d04547a5ace91cdf983e69a2