fix: added ipykernel to address 'No python3 ...' error

Author: moe-ad

requirements/requirements_docs.txt

@@ -3,11 +3,12 @@ enum-tools[sphinx]==0.12.0
 graphviz==0.20.1
 imageio==2.36.0
 imageio-ffmpeg==0.5.1
+ipykernel==6.29.5
 nbsphinx==0.9.5
 pypandoc==1.14
 pytest-sphinx==0.6.3
 pyvista==0.44.2
-sphinx==8.1.3
+sphinx==7.4.7
 sphinx-copybutton==0.5.2
 sphinx-gallery==0.18.0
 sphinx-notfound-page==1.0.4