docs: add developer guide for sgl-kernel (#3068)

sgl-kernel/developer_guide.md

+# Developer Guide for sgl-kernel
+
+## Development Environment Setup
+
+Use Docker to set up the development environment. See [Docker setup guide](https://github.com/sgl-project/sglang/blob/main/docs/developer/development_guide_using_docker.md#setup-docker-container).
+
+Create and enter development container:
+```bash
+docker run -itd --shm-size 32g --gpus all -v $HOME/.cache:/root/.cache --ipc=host --name sglang_zhyncs lmsysorg/sglang:dev /bin/zsh
+docker exec -it sglang_zhyncs /bin/zsh
+```
+
+## Project Structure
+
+### Dependencies
+
+Third-party libraries:
+
+- [CCCL](https://github.com/NVIDIA/cccl)
+- [CUTLASS](https://github.com/NVIDIA/cutlass)
+- [FlashInfer](https://github.com/flashinfer-ai/flashinfer)
+
+### Kernel Development
+
+Steps to add a new kernel:
+
+1. Implement in `sgl-kernel/src/sgl-kernel/csrc`
+2. Expose interface in `sgl-kernel/csrc/sgl_kernel_ops.cu` with pybind11
+3. Create Python wrapper in `sgl-kernel/src/sgl-kernel/ops/__init__.py`
+4. Expose Python interface in `sgl-kernel/src/sgl-kernel/__init__.py`
+
+### Build & Install
+
+Development build:
+
+```bash
+make build
+pip3 install dist/*whl --force-reinstall --no-deps
+# Or use: make install (runs pip install -e .)
+```
+
+### Testing & Benchmarking
+
+1. Add pytest tests in `sgl-kernel/tests/`
+2. Add benchmarks using [triton benchmark](https://triton-lang.org/main/python-api/generated/triton.testing.Benchmark.html) in `sgl-kernel/benchmark/`
+3. Run test suite