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

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

docs/design/index.md

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+# Design Documents
+
+This section contains design documents and architecture specifications for vLLM-omni.
+
+## Architecture Documents
+
+- [vLLM-omni Software Design Document](vllm_omni_design.md)
+- [Architecture Overview](architecture_overview.md)
+
+## API Design Documentation
+
+- [vLLM-omni API Documentation](api_design_doc.md)