docs: add description of tree property in Presentation for slide structure analysis

Author: ya7010

docs/docs/usage/index.md

@@ -10,3 +10,8 @@ By using the builder, you can create slides while utilizing slide masters and sl
 ```python
 --8<-- "codes/quick_start.py"
 ```
+
+Also, `tppt.Presentation` has a property called `tree`,
+which is a dictionary representing the tree structure of the presentation.
+
+It is used for automatic generation of slide master type definitions and can also be used to analyze how pptx files are structured.