[DOC] add LoRA user guide

[paulyu12]

What this PR does / why we need it?

Add LoRA user guide to DOC. The content refers to LoRA Adapters.

Does this PR introduce any user-facing change?

No

How was this patch tested?

No

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[wangxiyuan]

@paulyu12 Hi, thanks for the doc. I notice that most content is from vLLM doc. I think in vLLM Ascend doc, we don't need to re-write them again. For the most common part, we can add the link to vLLM directly. What we should write more is: what's the different usage on vLLM Ascend, for example, any model support list or limitation? Any feature + feature limitation? 2. How users use this feature with vLLM Ascend and so on.

[paulyu12]

Actually, users can use LoRA with the user guide of vLLM doc. No extra limitation as I know. Because aclgraph mode doesn't support LoRA until now, maybe I'll add some tips about how to fallback to non-aclgraph mode.

[paulyu12]

And the models that LoRA supports can be referred by vLLM Supported Models

[wangxiyuan]

So the doc can be simple in vllm-ascend. Just add something like:

vLLM Ascend support lora the same as vLLM. More detail can be found in vLLM official document.

Then add some limitation, like: does lora in vllm-ascend support graph mode? what model has been tested and so on.