### What this PR does / why we need it?
Add LoRA user guide to DOC. The content refers to [LoRA
Adapters](https://docs.vllm.ai/en/latest/features/lora.html).
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
No
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Signed-off-by: paulyu12 <507435917@qq.com>
