Pick out document build at CI (#337)

* Pick out document build at CI

* Explicit Python version for doc build as 3.12

.github/workflows/python-package.yml

@@ -22,10 +22,28 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
+        cache: 'pip'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         python -m pip install tox
     - name: Test with tox
       run: |
-        tox
+        tox run --skip-env doc
+
+  doc:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: "3.12"
+        cache: 'pip'
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        python -m pip install tox
+    - name: Test with tox
+      run: |
+        tox run -e doc