Add BottomSheetOverlay Explanation in Document's Overlays Section

[easyhooon]

https://slackhq.github.io/circuit/overlays/

Thank you for developing this fantastic library.

Having explanation about BottomSheetOverlay would be helpful for developers implementing Circuit in their projects, especially since bottom sheets are a commonly used UI pattern in Android applications.

Suggested content could include:

1. Explanation of BottomSheetOverlay concept and use cases(including <Model, Result>)
2. How to handle results from the bottom sheet
3. (optional) Explanation of the differences between Circuit BottomSheetOverlay and M3 ModalBottomSheet

I believe this addition would make the Circuit documentation more comprehensive and help developers better understand when and how to use Overlays in their applications.

[ZacSweers]

Doc improvement PRs are helpful but please keep them focused and without unnecessary AI-generated fluff.

[easyhooon]

In the documentation at https://slackhq.github.io/circuit/circuitx/, I can find explanations about BottomSheetOverlay. Since the usage examples are well documented, there seems to be no issues implementing it. Thanks :)