Create development guide for GatewayD

[mostafa]

• DevEnv setup, linting, make targets, go modules, etc.
• Package structure and components
• Configuration (global and plugins)
• Observability (logs, metrics and traces)
• Protobuf, gRPC and buf
• Schedulers
• Release process (test -> tag -> release)
• SDK and plugin development (plugin template, metrics merger, health check, etc.)