[Docs] Guide for using sleep mode and enable sleep mode (#660)

Signed-off-by: knlnguyen1802 <knlnguyen1802@gmail.com>
Co-authored-by: Jiangyun Zhu <riverclouds.zhu@qq.com>

docs/features/sleep_mode.md

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+# Sleep Mode
+
+vLLM-Omni’s **Sleep Mode** allows you to temporarily release most GPU memory used by a model—such as model weights and key-value (KV) caches (for autoregressive models)—**without stopping the server or unloading the Docker container**.
+
+This feature is inherited from [vLLM’s Sleep Mode](https://blog.vllm.ai/2025/10/26/sleep-mode.html), which provides zero-reload model switching for multi-model serving.  
+
+It is especially useful in **RLHF**, **training**, or **cost-saving scenarios**, where GPU resources must be freed between inference workloads.
+
+---
+
+## Omni Model
+
+Omni model inherit the feature from vLLM' Sleep Mode
+
+This means:
+
+- Support both Level 1 and Level 2 sleep, allow to release and reset both model weights and KV Cache
+
+## Diffusion Model Extension
+
+We added Sleep Mode support for **diffusion models**, which previously lacked this functionality.  
+In diffusion pipelines, this currently only offloads **model weight memory**, as these models typically do not use KV caches.
+
+This means:
+
+- Diffusion models can now enter Level 1 sleep.
+- Pipeline states (e.g., noise schedulers, buffers) remain intact after waking.
+- Useful for releasing VRAM between image generation or training cycles.
+
+---
+
+## Enable sleep mode
+To enable sleep mode, set the `enable_sleep_mode` in `engine_args` to `True`
+
+
+Example:
+```python
+omni = Omni(model=...,enable_sleep_mode=True)
+```