[Docs] Add qwen image missing doc in user guide (#111)

Signed-off-by: samithuang <285365963@qq.com>

docs/user_guide/examples/offline_inference/qwen_image.md

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+# Offline Inference Example of vLLM-Omni for Qwen-Image
+
+Source <https://github.com/vllm-project/vllm-omni/tree/main/examples/offline_inference/qwen_image>.
+
+
+## 🛠️ Installation
+
+Please refer to [installation](../../../getting_started/installation/README.md).
+
+## Run Offline inference with Qwen-Image
+
+First, navigate to the example folder
+```bash
+cd examples/offline_inference/qwen_image
+```
+
+This folder provides two simple entrypoints for experimenting with `Qwen/Qwen-Image` using vLLM-Omni:
+
+- `text_to_image.py`: command-line script for single image generation.
+- `gradio_demo.py`: lightweight Gradio UI for interactive prompt/seed/CFG exploration.
+
+### Command-line Usage
+
+```bash
+python text_to_image.py \
+  --prompt "a cup of coffee on the table" \
+  --seed 42 \
+  --cfg_scale 4.0 \
+  --num_images_per_prompt 1 \
+  --num_inference_steps 50 \
+  --height 1024 \
+  --width 1024 \
+  --output outputs/coffee.png
+```
+
+Key arguments:
+
+- `--prompt`: text description (string).
+- `--seed`: integer seed for deterministic sampling.
+- `--cfg_scale`: true CFG scale (model-specific guidance strength).
+- `--num_images_per_prompt`: number of images to generate per prompt (saves as `output`, `output_1`, ...).
+- `--num_inference_steps`: diffusion sampling steps (more steps = higher quality, slower).
+- `--height/--width`: output resolution (defaults 1024x1024).
+- `--output`: path to save the generated PNG.
+
+??? abstract "text_to_image.py"
+    ``````py
+    --8<-- "examples/offline_inference/qwen_image/text_to_image.py"
+    ``````
+
+> ℹ️ Qwen-Image currently publishes best-effort presets at `1328x1328`, `1664x928`, `928x1664`, `1472x1140`, `1140x1472`, `1584x1056`, and `1056x1584`. Adjust `--height/--width` accordingly for the most reliable outcomes.
+
+### Web UI Demo
+
+Launch the gradio demo:
+
+```bash
+python gradio_demo.py --port 7862
+```
+
+Then open `http://localhost:7862/` on your local browser to interact with the web UI.
+
+??? abstract "gradio_demo.py"
+    ``````py
+    --8<-- "examples/offline_inference/qwen_image/gradio_demo.py"
+    ``````