Add AI api details to "Using Google APIs" page

[parlough]

Package: https://pub.dev/packages/google_generative_ai Dedicated docs: https://ai.google.dev/tutorials/dart_quickstart dart.dev page: https://dart.dev/guides/google-apis

Should add a third, new subsection to the page with a little details about how Dart developers can interact with Google's AI APIs. Doesn't have to be long, perhaps just a cross-link or two with some details.