add package manifest to registration

src/Umbraco.Web.UI.Client/src/stories/extending/registration.stories.mdx

@@ -4,6 +4,16 @@ import { Meta } from '@storybook/addon-docs';
 
 # Registration
 
+## Package Manifest
+
+TODO: Describe the Package Manifest
+
+```json
+// show example of package-manifest.json
+```
+
+## UI Extension Registration
+
 Registering UI extensions happens through the global extension registry.
 
 There are two ways to register UI extensions:
@@ -11,7 +21,9 @@ There are two ways to register UI extensions:
 1. Reference an entry point resource in the manifest where full control over registration can take place.
 2. Directly through a manifest file
 
-## Entry Point Files
+### Entry Point Manifest
+
+TODO: describe where to position the entry point manifest file. Describe the UI extension manifest.
 
 **_Register an entry point in a JSON manifest_**
 
@@ -43,7 +55,9 @@ const manifest = {
 extensionRegistry.register(extension);
 ```
 
-## Manifest JSON files
+### JSON Manifest files
+
+TODO: describe how and where to position the JSON manifest files
 
 ```json
 {