docs: essentials docs update

docs/plugins/essentials/filesystem.md

@@ -3,3 +3,15 @@ uid: Plugins.Essentials.FileSystem
 ---
 
 # File System
+
+The `IFileSystem` interface from Prism Essentials provides a simple to use `DirectoryInfo` object for the AppData, Cache and Public file storage directories. From there is becomes a simple matter of normal File IO to locate, open and save files.
+
+You can additionally check if a file exists within an App Package:
+
+```cs
+if (await fileSystem.FileExistsAsync("myFile.txt"))
+{
+    using var stream = await fileSystem.OpenFileAsync("myFile.txt");
+    // Do something with the file stream.
+}
+```

docs/plugins/essentials/index.md

@@ -12,14 +12,33 @@ The API is generally inspired by a combination of Xamarin/.NET MAUI Essentials a
 
 Be sure to install `Prism.Plugin.Essentials.Maui`
 
+```cs
+builder.UseMauiApp<App>()
+    .UsePrism(prism => prism.UsePrismEssentials())
+```
+
 # [WPF](#tab/wpf)
 
 Be sure to install `Prism.Plugin.Essentials.Wpf`
 
+```cs
+protected override void RegisterTypes(IContainerRegistry containerRegistry)
+{
+    containerRegistry.UsePrismEssentials();
+}
+```
+
 # [Uno Platform](#tab/uno-platform)
 
 Be sure to install `Prism.Plugin.Essentials.Uno.WinUI`
 
+```cs
+protected override void RegisterTypes(IContainerRegistry containerRegistry)
+{
+    containerRegistry.UsePrismEssentials();
+}
+```
+
 ---
 
 > [!NOTE]