Add OverflowBar to Layout Widgets

### Page URL https://docs.flutter.dev/ui/widgets/layout ### Page source _No response_ ### Describe the problem I think that [OverflowBar](https://api.flutter.dev/flutter/widgets/OverflowBar-class.html) would fit into the `Multi-child layout widgets` section. ### Expected fix _No response_ ### Additional context _No response_ ### I would like to fix this problem. - [x] I will try and fix this problem on docs.flutter.dev.