[Doc] Add sphinx build for vllm-ascend

[Yikun]

What this PR does / why we need it?

This patch enables the doc build for vllm-ascend

• Add sphinx build for vllm-ascend
• Enable readthedocs for vllm-ascend
• Fix CI:
  • exclude vllm-empty/tests/mistral_tool_use to skip You need to agree to share your contact information to access this model which introduce in vllm-project/vllm@314cfad
  • Install test req to fix https://github.com/vllm-project/vllm-ascend/actions/runs/13304112758/job/37151690770:

    vllm-empty/tests/mistral_tool_use/conftest.py:4: in <module>
        import pytest_asyncio
    E   ModuleNotFoundError: No module named 'pytest_asyncio'
    

  • exclude docs PR

Does this PR introduce any user-facing change?

No

How was this patch tested?

1. test locally:

   # Install dependencies.
   pip install -r requirements-docs.txt
   
   # Build the docs and preview
   make clean; make html; python -m http.server -d build/html/

   Launch browser and open http://localhost:8000/.

2. CI passed with preview:
   https://vllm-ascend--55.org.readthedocs.build/en/55/