[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>

.github/workflows/vllm_ascend_test.yaml

@@ -25,6 +25,7 @@ on:
       - '*.txt'
       - '**/*.py'
       - '.github/workflows/vllm_ascend_test.yaml'
+      - '!docs/**'
   pull_request:
     branches:
       - "main"
@@ -32,6 +33,7 @@ on:
       - '*.txt'
       - '**/*.py'
       - '.github/workflows/vllm_ascend_test.yaml'
+      - '!docs/**'
 
 # Bash shells do not use ~/.profile or ~/.bashrc so these shells need to be explicitly
 # declared as "shell: bash -el {0}" on steps that need to be properly activated.