feat: Document how to produce a Windows MSIX file

[bryanoltman]

Description

To submit an app to the Windows Store, the developer needs to create an MSIX file. package:msix is a popular way of doing this, but it rebuilds the Flutter app with non-Shorebird Flutter, which causes problems.

To get around this, one can update the msix_config section in the pubspec.yaml to include build_windows: false, which will cause package:msix to use an existing build, which should be produced by shorebird release windows.

This issue stemmed from https://discord.com/channels/1030243211995791380/1332089260605444187

[bryanoltman]

This may be at home in a new "submitting" guide for the Windows Store as an addition to the play store and app store guides.