Adding a Flutter-approved guide on Semantics

[shihaohong]

Proposal

I believe that it could be helpful to have a guide on Flutter's Semantics and how to properly use it. From the API docs, it can be difficult to understand and requires jumping into source code and figuring it out how to properly build out the Semantic tree to adhere to accessibility.

I found two third-party articles about it and a Stack Overflow question, but not much else.

1. https://stackoverflow.com/questions/51465328/flutter-semantics-explanation
2. https://www.didierboelens.com/2018/07/semantics/
3. https://medium.com/flutter-community/a-deep-dive-into-flutters-accessibility-widgets-eb0ef9455bc

It could be a cookbook or a comprehensive addition to the API docs.