[Docs] Add msix embedding instructions (#13489)

doc/devdocs/embedded-msix.md

@@ -0,0 +1,30 @@
+# How to integrate new MSIX module with PowerToys Settings and WiX installer
+[`issue_11705_with_example` branch](https://github.com/microsoft/PowerToys/tree/issue_11705_with_example) contains HelloWorld UWP C# MSIX application which reads its module settings and is installed along PowerToys.
+
+## Steps
+- uncomment everything near "TODO: Use to activate embedded MSIX" comments
+- build PowerToys solution
+- deploy HelloModule module and sign it with a self-signed certificate (don't forget to put it into a trusted store)
+- build PowerToysSetup solution and install it
+
+Type HelloModule into start search and observe that it was installed:
+
+<img src="../images/msix/hello-module-start.png">
+
+Open PowerToys settings and change the stub setting to something:
+
+<img src="../images/msix/hello-module-settings-page.png">
+
+Open HelloModule:
+
+<img src="../images/msix/hello-module-screen.png">
+
+First time you press "Load Settings", it'll detect that it doesn't have permissions to access local file system and open its system settings window, toggle it there:
+
+<img src="../images/msix/hello-module-app-permissions.png">
+
+(it's a known uwp limitation, see https://stackoverflow.com/a/53533414/657390)
+
+HelloModule will close then, restart it, press "Load Settings" and you should see that the application was able to load the setting string which was set by the settings app:
+
+<img src="../images/msix/hello-module-loaded-settings.png">