Create development guide for GatewayD #3
Description
- 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.)