docs: Add guidelines for implementing WinUI/WinRT features

doc/articles/contributing/guidelines/implementing-a-new winui-winrt-feature.md

@@ -0,0 +1,31 @@
+## Guidelines for implementing a new WinUI/WinRT API
+
+Implementing a new WinUI/WinRT API generally requires to:
+- Find the generated API source file (e.g. `src\Uno.UWP\Generated\3.0.0.0\Windows.Data.Pdf\PdfDocument.cs`)
+- Copy the file to the non-generated location (e.g. `src\Uno.UWP\Data.Pdf\PdfDocument.cs`)
+- Keep only the members that need to be implemented in the non-generated location
+- Remove (completely or partially depending on the platforms) the implemented members in the generated file
+
+If your API implementation is for a specific platform:
+- You can use a platform suffix in the source file name (`PdfDocument.android.cs`) so the file is built only for this platform
+- Remove the parts that relate to your platform in the `NotImplemented` attribute:
+    ```
+    #if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+	[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "NET461", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__", "__MACOS__")]
+	public  global::Windows.Data.Pdf.PdfPageDimensions Dimensions
+	{
+	#endif
+    ```
+    becomes
+    ```
+    #if false || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+	[global::Uno.NotImplemented("__IOS__", "NET461", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__", "__MACOS__")]
+	public  global::Windows.Data.Pdf.PdfPageDimensions Dimensions
+	{
+	#endif
+    ```
+    when implemented for Android only.
+
+When implementing a feature, try to place as much code as possible in a common source file (a non-suffixed file), so that it is reused across platforms. Make sure to follow [partial classes coding guidelines](code-style.md).
+
+Note that the generated files may be overriden at any point, and placing custom code (aside from changing the `NotImplemented` values) will be overwritten.

doc/articles/toc.yml

@@ -446,6 +446,8 @@
       href: contributing/guidelines/updating-dependencies.md
     - name: Guidelines for issue triage
       href: contributing/guidelines/issue-triage.md
+    - name: Guidelines for implementing a new WinUI/WinRT feature
+      href: contributing/guidelines/implementing-a-new winui-winrt-feature.md
     - name: Adding documentation
       href: uno-development/doc-on-docs.md
     - name: DocFX

doc/articles/uno-development/building-uno-ui.md

@@ -75,3 +75,18 @@ The versions used are centralized in the [Directory.Build.targets](https://githu
 locations where `<PackageReference />` are used.
 
 When updating the versions of nuget packages, make sure to update all the .nuspec files in the [Build folder](https://github.com/unoplatform/uno/tree/master/build).
+
+### Running the SyncGenerator tool
+
+Uno Platform uses a tool which synchronizes all WinRT and WinUI APIs with the type implementations already present in Uno. This ensures that all APIs are present for consumers of Uno, even if some are not implemented.
+
+The synchronization process takes the APIs provided by the WinMD files referenced by the `Uno.UWPSyncGenerator.Reference` project and generates stubbed classes for types in the `Generated` folders of Uno.UI, Uno.Foundation and Uno.WinRT projects. If the generated classes have been partially implemented in Uno (in the non-Generated folders), the tool will automatically skip those implemented methods.
+
+The tool needs to be run on Windows because of its dependency on the Windows SDK WinMD files.
+
+To run the synchronization tool:
+- Open a `Developer Command Prompt for Visual Studio` (2019 or 2022)
+- Go the the `uno\build` folder (not the `uno\src\build` folder)
+- Run the `run-api-sync-tool.cmd` script; make sure to follow the instructions
+
+Note that as of Uno 3.10, the tool is manually run for the UWP part of the build and automatically run as part of the CI during the WinUI part of the build.

doc/articles/uno-development/contributing-intro.md

@@ -26,6 +26,10 @@ Whether you're fixing a bug or working on a new feature, [inspecting the visual
 
 See [Uno's code conventions and common patterns here](../contributing/guidelines/code-style.md).
 
+## Implementing a new feature
+
+See how to implement a new [feature here](../contributing/guidelines/implementing-a-new winui-winrt-feature.md).
+
 ## Adding tests
 
 Uno's stability rests upon a comprehensive testing suite. A code contribution usually isn't complete without a test.