docs: add example script for visualizing presentation tree structure

Author: ya7010

docs/docs/codes/pptx_tree.py

@@ -0,0 +1,5 @@
+import json
+
+import tppt
+
+print(json.dumps(tppt.Presentation("your.pptx").tree, indent=2, ensure_ascii=False))

docs/docs/usage/index.md

@@ -14,4 +14,8 @@ By using the builder, you can create slides while utilizing slide masters and sl
 Also, `tppt.Presentation` has a property called `tree`,
 which is a dictionary representing the tree structure of the presentation.
 
+```python
+--8<-- "codes/pptx_tree.py"
+```
+
 It is used for automatic generation of slide master type definitions and can also be used to analyze how pptx files are structured.