Feature request: accesskit_consumer API documentation

Hi! Very cool project, really glad this exists and eager to learn more about how to interact with the accesskit_consumer API. I'm using egui and have been exploring its integration with accesskit and am pretty impressed with the out-of-the-box experience with Voiceover in my app.

I've started poking at the source a bit, but haven't quite found the right entry point to do anything useful yet and was hoping to get pointed at a good place to start.

At a high level what I think needs to happen is something like:

• Get a reference to a window that provides accesskit metadata
• Implement a handler to receive accessibility tree updates from the window
• Do something with the update

If it's welcomed, I'd be happy to document what I learn for the project so it's easier for the next person. Thank you!

Start by locating the public accesskit_consumer API and the egui/accesskit integration entry points; the issue does not name specific files or tests. Document how to obtain a window reference, receive accessibility tree updates, and use those updates, with the relevant API flow clear enough for a new user to follow.