[doc] Document how to do build.dart that downloads a binary

[dcharkes]

Or maybe even add an example? (But where do we host the binaries for the example?)

Context:

• Use native-assets to vendor MediaPipe SDK google/flutter-mediapipe#9

[Piinks]

where do we host the binaries for the example?

Could we pick one from CIPD just for the purposes of an example? For the framework we download https://chrome-infra-packages.appspot.com/p/skia/tools/goldctl for every commit in infra to run golden file tests

[dcharkes]

Some example: #996