add a diagram to introduce workflow of casting video player (#31707)

* add diagram

* modify name

* change the diagram.

* Modify according to comments.

Author: nicelyjust

examples/tv-casting-app/APIs.md

@@ -67,6 +67,9 @@ Next, you're ready to:
 1. [Read](#read-operations) endpoint attributes like playback state.
 1. [Subscribe](#subscriptions) to playback events.
 
+In order to illustrate these steps, refer to the figure below
+![workflow of casting video player](./diagram/workflow_of_casting_video_player.png)
+
 ## Build and Setup
 
 The Casting Client is expected to consume the Matter TV Casting library built