Add extensions tab (#1488)

docs/index.md

@@ -52,10 +52,16 @@ quickstart/custom_dataset
 quickstart/viewer_quickstart
 quickstart/export_geometry
 quickstart/data_conventions
-quickstart/blender_addon
 Contributing<reference/contributing>
 ```
 
+```{toctree}
+:hidden:
+:caption: Extensions
+
+extensions/blender_addon
+```
+
 ```{toctree}
 :hidden:
 :caption: NeRFology