docs(hugr-py): expand toctree (#1411)

now looks like: 
<img width="304" alt="image"
src="https://github.com/user-attachments/assets/ed030671-e933-48b0-bdf9-1ace4b601f75">

hugr-py/docs/api-docs/conf.py

@@ -27,6 +27,7 @@
         "image_light": "_static/Quantinuum_logo_black.png",
         "image_dark": "_static/Quantinuum_logo_white.png",
     },
+    "show_navbar_depth": 2,
 }
 
 html_static_path = ["../_static"]