Add Flutter file structure information

[leighajarett]

What information is missing?

When I went through the Getting Started pages for Flutter, I didn't see any explanation for the file structure or what information lives in what folder. I think some page that walks through this info, linked from the Getting Started pages, would be super helpful!

Additionally, when I went to go and add Firebase to my project, I wasn't sure where the Firebase app code should live. Some information on how Flutter <> Firebase files should be ideally structured would be great.

[sfshaza2]

We do document a typical flutter package but it's rather buried. In the short term, we could link to that info: https://docs.flutter.dev/development/packages-and-plugins/developing-packages#step-1-create-the-package