[Doc] Add sphinx build for vllm-ascend (#55)

### 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
314cfade02
- 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:
    ```bash
    # 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/

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

docs/requirements-docs.txt

@@ -0,0 +1,8 @@
+sphinx==6.2.1
+sphinx-argparse==0.4.0
+sphinx-book-theme==1.0.1
+sphinx-copybutton==0.5.2
+sphinx-design==0.6.1
+sphinx-togglebutton==0.3.2
+myst-parser==3.0.1
+msgspec