Comprehensive contribution guide for developers

[cwing-nvidia]

Description:
Create detailed guide for all types of NeMo Gym contributors with setup, requirements, and best practices.

Scope:

• Types of contributions (environments, integrations, etc.)
• Development setup and commands
• CI/CD requirements and troubleshooting
• DCO and commit signing setup
• Common issues and solutions

Acceptance Criteria:

• "Types of Contributions" section with priorities
• Complete development setup instructions
• DCO and commit signing guide with troubleshooting
• CI/CD requirements (build docs, copyright, etc.)
• Common issues and FAQ section