Add documentation to make it easier for developers to use

[mariamhas]

Add more specific documentation regarding the features and how to best to leverage JNIgen within their Dart and Flutter applications.

The documentation should include:

• example for interface implementation
• Add a more specific add to app example

[mariamhas]

we should include in our docs how someone could do manually set that the calls generated are async. from the 1P customer perspective... having this as a boolean would make it easier but for mvp we should alteast have a way for developers to do it manually.

[HosseinYousefi]

Closed in favor of https://github.com/dart-lang/native/issues/1526