docs: move over utility api migration to frontend plugin

Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

docs/frontend-system/building-plugins/migrating.md

@@ -69,20 +69,94 @@ TODO
 
 ## APIs
 
-Let's imagine we have the following API:
+There are a few things to keep in mind in regards to utility APIs.
 
-```ts
-const myApi = createApiFactory(myApiRef, new SampleMyApi());
-```
+### React package interface and ref changes
 
-you can transform the API above to an extension using the `createApiExtension` creator:
+Let's begin with [your `-react` package](../../architecture-decisions/adr011-plugin-package-structure.md). The act of exporting TypeScript interfaces and API refs have not changed from the old system. You can typically keep those as-is. For illustrative purposes, this is an example of an interface and its API ref:
 
-```ts
-export const myApi = createApiExtension({
-  factory: createApiFactory(myApiRef, new SampleMyApi()),
+```tsx title="in @internal/plugin-example-react"
+import { createApiRef } from '@backstage/frontend-plugin-api';
+
+/**
+ * Performs some work.
+ * @public
+ */
+export interface WorkApi {
+  doWork(): Promise<void>;
+}
+
+/**
+ * The work interface for the Example plugin.
+ * @public
+ */
+export const workApiRef = createApiRef<WorkApi>({
+  id: 'plugin.example.work',
 });
 ```
 
+In this example, the plugin ID already follows the [Frontend System Naming Patterns](../architecture/naming-patterns). If it doesn't, you may want to consider renaming that ID at this point. Don't worry, this won't hurt consumers in the old frontend system since the ID is mostly used for debugging purposes there. In the new system, it's much more important and appears in app-config files and similar.
+
+Note at the top of the file that it uses the updated import from `@backstage/frontend-plugin-api` that we migrated in the previous section, instead of the old `@backstage/core-plugin-api`.
+
+### Plugin package changes
+
+Now let's turn to the main plugin package where the plugin itself is exported. You will probably already have a `createPlugin` call in here. Before we changed the `core-plugin-api` imports it'll have looked somewhat similar to the following:
+
+```tsx title="in @internal/plugin-example, NOTE THIS IS LEGACY CODE"
+import {
+  storageApiRef,
+  createPlugin,
+  createApiFactory,
+} from '@backstage/core-plugin-api';
+import { workApiRef } from '@internal/plugin-example-react';
+import { WorkImpl } from './WorkImpl';
+
+const exampleWorkApi = createApiFactory({
+  api: workApiRef,
+  deps: { storageApi: storageApiRef },
+  factory: ({ storageApi }) => new WorkImpl({ storageApi }),
+});
+
+/** @public */
+export const catalogPlugin = createPlugin({
+  id: 'example',
+  apis: [exampleWorkApi],
+});
+```
+
+The major changes we'll make are
+
+- Optionally change the old imports to the new package as per the top section of this guide
+- Wrap the existing API factory in a `createApiExtension`
+- Change to the new version of `createPlugin` which exports this extension
+- Change the plugin export to be the default instead
+
+The end result, after simplifying imports and cleaning up a bit, might look like this:
+
+```tsx title="in @internal/plugin-example"
+import {
+  storageApiRef,
+  createPlugin,
+  createApiFactory,
+  createApiExtension,
+} from '@backstage/frontend-plugin-api';
+import { workApiRef } from '@internal/plugin-example-react';
+import { WorkImpl } from './WorkImpl';
+
+const exampleWorkApi = createApiExtension({
+  factory: createApiFactory({
+    api: workApiRef,
+    deps: { storageApi: storageApiRef },
+    factory: ({ storageApi }) => new WorkImpl({ storageApi }),
+  }),
+});
+```
+
+### Further work
+
+Since utility APIs are now complete extensions, you may want to take a bigger look at how they used to be used, and what the new frontend system offers. You may for example consider [adding configurability or inputs](../utility-apis/02-creating.md) to your API, if that makes sense for your current application.
+
 ## Plugin
 
 In the legacy frontend system a plugin was defined in its own `plugin.ts` file as following:
@@ -92,7 +166,7 @@ In the legacy frontend system a plugin was defined in its own `plugin.ts` file a
 
   export const myPlugin = createPlugin({
     id: 'my-plugin',
-    apis: [myApi],
+    apis: [exampleWorkApi],
     routes: {
       ...
     },
@@ -112,7 +186,7 @@ The new `createPlugin` function doesn't accept apis anymore as apis are now exte
     id: 'my-plugin',
     // bind all the extensions to the plugin
     /* highlight-next-line */
-    extensions: [homePage, myApi],
+    extensions: [homePage, exampleWorkApi],
     routes: {
       ...
     },