[Docs] optimize and supplement docs system (#86)

Signed-off-by: Gaohan123 <gaohan19@huawei.com>

docs/getting_started/quickstart.md

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+# Quickstart
+
+This guide will help you quickly get started with vLLM-Omni to perform:
+
+- Offline batched inference
+- Online serving using OpenAI-compatible server
+
+## Prerequisites
+
+- OS: Linux
+- Python: 3.12
+
+## Installation
+
+Please refer to [installation](installation/README.md)
+
+## Offline Inference
+
+Please refer to [offline inference](../user_guide/examples/offline_inference/qwen2_5_omni.md)
+
+## Online Serving with OpenAI-Completions API
+
+Please refer to [online serving](../user_guide/examples/online_serving/README.md)