[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/source/developer_guide/contributing.md

@@ -0,0 +1,107 @@
+# Contributing
+
+## Building and testing
+It's recommended to set up a local development environment to build and test
+before you submit a PR.
+
+### Prepare environment and build
+
+Theoretically, the vllm-ascend build is only supported on Linux because
+`vllm-ascend` dependency `torch_npu` only supports Linux.
+
+But you can still set up dev env on Linux/Windows/macOS for linting and basic
+test as following commands:
+
+```bash
+# Choose a base dir (~/vllm-project/) and set up venv
+cd ~/vllm-project/
+python3 -m venv .venv
+source ./.venv/bin/activate
+
+# Clone vllm code and install
+git clone https://github.com/vllm-project/vllm.git
+cd vllm
+pip install -r requirements-build.txt
+VLLM_TARGET_DEVICE="empty" pip install .
+cd ..
+
+# Clone vllm-ascend and install
+git clone https://github.com/vllm-project/vllm-ascend.git
+cd vllm-ascend
+pip install -r requirements-dev.txt
+
+# Then you can run lint and mypy test
+bash format.sh
+
+# Build:
+# - only supported on Linux (torch_npu available)
+# pip install -e .
+# - build without deps for debugging in other OS
+# pip install -e . --no-deps
+
+# Commit changed files using `-s`
+git commit -sm "your commit info"
+```
+
+### Testing
+
+Although vllm-ascend CI provide integration test on [Ascend](https://github.com/vllm-project/vllm-ascend/blob/main/.github/workflows/vllm_ascend_test.yaml), you can run it
+locally. The simplest way to run these integration tests locally is through a container:
+
+```bash
+# Under Ascend NPU environment
+git clone https://github.com/vllm-project/vllm-ascend.git
+cd vllm-ascend
+
+IMAGE=vllm-ascend-dev-image
+CONTAINER_NAME=vllm-ascend-dev
+DEVICE=/dev/davinci1
+
+# The first build will take about 10 mins (10MB/s) to download the base image and packages
+docker build -t $IMAGE -f ./Dockerfile .
+# You can also specify the mirror repo via setting VLLM_REPO to speedup
+# docker build -t $IMAGE -f ./Dockerfile . --build-arg VLLM_REPO=https://gitee.com/mirrors/vllm
+
+docker run --name $CONTAINER_NAME --network host --device $DEVICE \
+           --device /dev/davinci_manager --device /dev/devmm_svm \
+           --device /dev/hisi_hdc -v /usr/local/dcmi:/usr/local/dcmi \
+           -v /usr/local/bin/npu-smi:/usr/local/bin/npu-smi \
+           -v /usr/local/Ascend/driver/lib64/:/usr/local/Ascend/driver/lib64/ \
+           -ti --rm $IMAGE bash
+
+cd vllm-ascend
+pip install -r requirements-dev.txt
+
+pytest tests/
+```
+
+## DCO and Signed-off-by
+
+When contributing changes to this project, you must agree to the DCO. Commits must include a `Signed-off-by:` header which certifies agreement with the terms of the DCO.
+
+Using `-s` with `git commit` will automatically add this header.
+
+## PR Title and Classification
+
+Only specific types of PRs will be reviewed. The PR title is prefixed appropriately to indicate the type of change. Please use one of the following:
+
+- `[Attention]` for new features or optimization in attention.
+- `[Communicator]` for new features or optimization in communicators.
+- `[ModelRunner]` for new features or optimization in model runner.
+- `[Platform]` for new features or optimization in platform.
+- `[Worker]` for new features or optimization in worker.
+- `[Core]` for new features or optimization  in the core vllm-ascend logic (such as platform, attention, communicators, model runner)
+- `[Kernel]` changes affecting compute kernels and ops.
+- `[Bugfix]` for bug fixes.
+- `[Doc]` for documentation fixes and improvements.
+- `[Test]` for tests (such as unit tests).
+- `[CI]` for build or continuous integration improvements.
+- `[Misc]` for PRs that do not fit the above categories. Please use this sparingly.
+
+> [!NOTE]
+> If the PR spans more than one category, please include all relevant prefixes.
+
+## Others
+
+You may find more information about contributing to vLLM Ascend backend plugin on [<u>docs.vllm.ai</u>](https://docs.vllm.ai/en/latest/contributing/overview.html).
+If you find any problem when contributing, you can feel free to submit a PR to improve the doc to help other developers.