[Apple Silicon] Add Metal kernel support in sgl-kernel (#23449)

Signed-off-by: Xiaodong Ye <yeahdongcn@gmail.com>

docs_new/docs/hardware-platforms/apple_metal.mdx

@@ -6,6 +6,19 @@ metatags:
 
 This document describes how run SGLang on Apple Silicon using [Metal (MLX)](https://opensource.apple.com/projects/mlx/). If you encounter issues or have questions, please [open an issue](https://github.com/sgl-project/sglang/issues).
 
+## Prerequisites
+
+Building the native Metal kernels in `sgl-kernel` requires the Apple
+toolchain (`clang++`, the Metal framework headers, and `xcrun`). These ship
+with the **Xcode Command Line Tools**, which cannot be installed via `pip`:
+
+```bash
+xcode-select --install
+```
+
+If you have the full Xcode app installed, the Command Line Tools are already
+available. You can verify with `xcode-select -p && xcrun --find metal`.
+
 ## Install SGLang
 
 You can install SGLang using one of the methods below.
@@ -17,8 +30,15 @@ You can install SGLang using one of the methods below.
 git clone https://github.com/sgl-project/sglang.git
 cd sglang
 
-# Install sglang python package
-pip install --upgrade pip
+# Create and activate a virtual environment
+uv venv -p 3.12 sglang-metal
+source sglang-metal/bin/activate
+
+# (Optional) Compile sgl-kernel
+uv pip install --upgrade pip
+uv run sgl-kernel/setup_metal.py install
+
+# Install sglang python package along with diffusion support
 rm -f python/pyproject.toml && mv python/pyproject_other.toml python/pyproject.toml
 uv pip install -e "python[all_mps]"
 ```

docs_new/docs/hardware-platforms/mthreads_gpu.mdx

@@ -22,7 +22,7 @@ pip install --upgrade pip
 cd sgl-kernel
 python setup_musa.py install
 
-# Install sglang python package
+# Install sglang python package along with diffusion support
 cd ..
 rm -f python/pyproject.toml && mv python/pyproject_other.toml python/pyproject.toml
 pip install -e "python[all_musa]"

docs_new/docs/sglang-diffusion/installation.mdx

@@ -120,7 +120,7 @@ git clone https://github.com/sgl-project/sglang.git
 cd sglang
 
 # Create and activate a virtual environment
-uv venv -p 3.11 sglang-diffusion
+uv venv -p 3.12 sglang-diffusion
 source sglang-diffusion/bin/activate
 
 # Install the Python packages