add test case for sphinx<9

Author: rocco8773

.github/workflows/documentation.yml

@@ -63,6 +63,10 @@ jobs:
           os: ubuntu-latest
           sphinx: sphinx<8.2
 
+        - python: '3.13'
+          os: ubuntu-latest
+          sphinx: sphinx<9.0
+
     steps:
     - name: Checkout code
       uses: actions/checkout@v3