Moved the template to .github and added checkboxes for platforms.

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -6,9 +6,9 @@
 
 **Applies to the following platforms:**
 
-| UWP              | WPF              | WinUI            |
-| :--------------- | :--------------- | :----------------|
-| <!-- Yes/No? --> | <!-- Yes/No? --> | <!-- Yes/No? --> |
+- [] WinUI
+- [] WPF
+- [] UWP
 
 **Anything that requires particular review or attention?**