[Doc][Model] Add principles of modeling files in vllm-ascend

[shen-shanshan]

What this PR does / why we need it?

Add principles of modeling files in vllm-ascend.

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.11.0
• vLLM main: vllm-project/vllm@2918c1b

[github-actions]

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[gemini-code-assist]

Code Review

This pull request adds a README.md file to the vllm_ascend/models directory. This documentation clarifies the project's strategy to move away from maintaining separate modeling files, opting instead to register and replace CustomOp in vllm to support model inference on Ascend devices. This is a documentation-only change that improves clarity on the project's architecture. The change is straightforward and I have no high or critical severity feedback.

[shen-shanshan]

CC @Yikun