frontend-app-api: extract createApp out into frontend-defaults

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/frontend-system/architecture/10-app.md

@@ -16,7 +16,7 @@ Below is a simple example of how to create and render an app instance:
 
 ```ts
 import ReactDOM from 'react-dom/client';
-import { createApp } from '@backstage/frontend-app-api';
+import { createApp } from '@backstage/frontend-defaults';
 
 // Create your app instance
 const app = createApp({

docs/frontend-system/architecture/25-extension-overrides.md

@@ -295,7 +295,7 @@ export default createExtensionOverrides({
 Assuming the above code resides in the `@internal/search-page` package, you can install it in your app like this:
 
 ```tsx title="packages/app/src/App.tsx"
-import { createApp } from '@backstage/frontend-app-api';
+import { createApp } from '@backstage/frontend-defaults';
 import searchPageOverride from '@internal/search-page';
 
 const app = createApp({

docs/frontend-system/architecture/36-routes.md

@@ -256,7 +256,7 @@ app:
 We also have the ability to express this in code as an option to `createApp`, but you of course only need to use one of these two methods:
 
 ```tsx title="packages/app/src/App.tsx"
-import { createApp } from '@backstage/frontend-app-api';
+import { createApp } from '@backstage/frontend-defaults';
 import catalog from '@backstage/plugin-catalog';
 import scaffolder from '@backstage/plugin-scaffolder';