Document Windows build path breaking change (#9339)

_Description of what this PR is changing or adding, and why:_
To support Windows Arm64, we updated the Windows's build directory to
include the target architecture. Previously, the Windows build assumed
an x64 build.

Example build path before:
`build/windows/runner/Release/hello_world.exe`
Example build path after:
`build/windows/x64/runner/Release/hello_world.exe`

Tooling that depends on this build path will need to be updated. For
example, `package:msix` takes your app's executable and bundles for the
Microsoft Store. It needed to be updated:
YehudaKremer/msix#205

_Issues fixed by this PR (if any):_
flutter/flutter#129804

/cc @pbo-linaro Who implemented this.

## Presubmit checklist

- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
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- [x] This PR uses [semantic line
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src/release/breaking-changes/index.md

@@ -37,13 +37,15 @@ release, and listed in alphabetical order:
 * [Deprecate `describeEnum` and update `EnumProperty` to be type strict][]
 * [Deprecated just-in-time navigation pop APIs for Android Predictive Back][]
 * [Updated default text styles for menus][]
+* [Windows build path changed to add the target architecture][]
 
 [Customize tabs alignment using the new `TabBar.tabAlignment` property]: {{site.url}}/release/breaking-changes/tab-alignment
 [Deprecate `textScaleFactor` in favor of `TextScaler`]: {{site.url}}/release/breaking-changes/deprecate-textscalefactor
 [Deprecate TextField.canRequestFocus]: {{site.url}}/release/breaking-changes/can-request-focus
 [Deprecate `describeEnum` and update `EnumProperty` to be type strict]: {{site.url}}/release/breaking-changes/describe-enum
 [Deprecated just-in-time navigation pop APIs for Android Predictive Back]: {{site.url}}/release/breaking-changes/android-predictive-back
 [Updated default text styles for menus]: {{site.url}}/release/breaking-changes/menus-text-style
+[Windows build path changed to add the target architecture]: {{site.url}}/release/breaking-changes/windows-build-architecture
 
 ### Released in Flutter 3.13
 

src/release/breaking-changes/windows-build-architecture.md

@@ -0,0 +1,62 @@
+---
+title: Windows build path changed to add the target architecture
+description: >
+  In preparation for supporting Windows Arm64, 
+  the Windows build path was updated to include the target architecture.
+---
+
+## Summary
+
+Built executables for Flutter Windows apps are now located in architecture
+dependent folders
+
+## Context
+
+In preparation for supporting Windows Arm64, the Windows build path was
+updated to add the build's target architecture.
+
+Previously, Flutter builds for Windows assumed an x64 target architecture.
+
+## Migration guide
+
+You may need to update your infrastructure to use the new Flutter Windows
+build path.
+
+Example build path before the migration:
+
+```
+build\windows\runner\Release\hello_world.exe
+```
+
+Example build path after the migration if targeting x64:
+
+```
+build\windows\x64\runner\Release\hello_world.exe
+```
+
+Example build path after the migration if targeting Arm64:
+
+```
+build\windows\arm64\runner\Release\hello_world.exe
+```
+
+If you use [`package:msix`][], update to version 3.15.0 or newer.
+
+[`package:msix`]: {{site.pub-pkg}}/msix
+
+## Timeline
+
+Landed in version: TBD
+
+In stable release: TBD
+
+## References
+
+Design document:
+*  [https://flutter.dev/go/windows-arm64][]
+
+Relevant pull requests:
+* [Introduce architecture subdirectory for Windows build][]
+
+[https://flutter.dev/go/windows-arm64]: {{site.main-url}}/go/windows-arm64
+[Introduce architecture subdirectory for Windows build]: {{site.github}}/flutter/flutter/pull/131843