add sphinx_rtd_theme to sphinx workflow

.github/workflows/sphinx.yml

@@ -9,6 +9,12 @@ jobs:
       contents: write
     steps:
     - uses: actions/checkout@v4
+    - name: Set up Conda
+      uses: conda-incubator/setup-miniconda@v2
+      with:
+        auto-activate-base: true
+    - name: Install dependencies
+      run: conda install -y sphinx_rtd_theme
     - name: Build HTML
       uses: ammaraskar/sphinx-action@master
     - name: Upload artifacts