[supply] Better document metadata folder structure

[janpio]

See https://stackoverflow.com/q/57001657/252627
We should probably have a tree like output on https://docs.fastlane.tools/actions/supply/ that shows an example folder structure, so it is easier than having to already have screenshots uploaded and then use fastlane supply init to create that structure.

[kg-currenxie]

Yea... We already have custom made screenshots. How do I put them in the screenshots folder to satisfy Apple? They (currently) require screenshots for 5.5, 6.5, and 12.9 inch devices. And we have them in different languages. 🤷 I assume the order they appear in the App Store is alphabetical by file name?

https://docs.fastlane.tools/actions/supply/#images-and-screenshots nor https://docs.fastlane.tools/getting-started/ios/screenshots/ help much