Add detailed developer documentation and contribution guides.

[RedZapdos123]

Summary:

This PR significantly enhances FastReID's documentation and developer experience by adding deatiled guides for contributors, developers, and users. This makes the project more accessible to new contributors and provides clear guidance for extending FastReID.

Key Changes:

Contribution Guidelines:

Complete contribution guidelines have been added through CONTRIBUTING.md. Stepwise development environment setup instructions are now available. Clear coding standards and best practices have been documented. Detailed testing guidelines have been provided.

Developer Architecture Guide:

An in-depth architecture overview has been implemented in DEVELOPER_GUIDE.md. Extension examples showing how to add datasets, backbones, and losses have been included. Understanding of FastReID's modular design through the registry system has been documented. Guidelines for efficient code and performance optimization have been provided.

Enhanced Documentation:

A detailed documentation for VeRiWild model usage has been added, for creating a detailed example of model usage. A complete codebase index and structure documentation has been created. Better documentation of public APIs has been implemented. Community guidelines for respectful collaboration have been established.

Benefits:

1. New contributors can get started quickly with lower barriers to entry.
2. Clear guidelines lead to consistent code and better code quality.
3. Examples accelerate feature development for faster development cycles.
4. Welcoming documentation encourages participation and community growth.
5. Architectural insights help developers understand the codebase for better knowledge sharing.

Documentation Structure:

The enhanced documentation includes CONTRIBUTING.md for contribution guidelines. DEVELOPER_GUIDE.md provides architecture and extension guidance. README_FOR_DEVELOPERS.md offers comprehensive developer information. VERIWILD_MODEL_GUIDE.md contains specific model usage instructions. CODEBASE_INDEX.md documents the complete project structure.

Key Sections:

For contributors, development environment setup instructions are provided. Code style and testing guidelines have been documented. The pull request process has been outlined. Community standards have been established.

For developers, project architecture overview is available. The registry based extension system has been explained. Performance optimization tips have been included. Debugging and profiling guidance has been provided.

For users, migration guides for new versions are available. Feature overview and benefits have been documented. Usage examples and best practices have been included.

Impact:

This documentation update will increase contributions by lowering barriers for new contributors. Code quality will improve through consistent standards and practices. Development will be accelerated with clear examples and guidelines. The community will grow with welcoming and inclusive documentation. The support burden will be reduced through self-service documentation for common questions.

The enhanced documentation makes FastReID more accessible to researchers, developers, and contributors at all levels.