docs: remove references to @backstage/core

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

docs/plugins/composability.md

@@ -511,10 +511,11 @@ clarify intent. Refer to the following table to formulate the new name:
 ## Porting Existing Apps
 
 The first step of porting any app is to replace the root `Routes` component with
-`FlatRoutes` from `@backstage/core`. As opposed to the `Routes` component,
-`FlatRoutes` only considers the first level of `Route` components in its
-children, and provides any additional children to the outlet of the route. It
-also removes the need to append `"/*"` to paths, as it is added automatically.
+`FlatRoutes` from `@backstage/core-app-api`. As opposed to the `Routes`
+component, `FlatRoutes` only considers the first level of `Route` components in
+its children, and provides any additional children to the outlet of the route.
+It also removes the need to append `"/*"` to paths, as it is added
+automatically.
 
 ```diff
 const AppRoutes = () => (

docs/plugins/plugin-development.md

@@ -36,7 +36,7 @@ to avoid import cycles, for example like this:
 
 ```tsx
 /* src/routes.ts */
-import { createRouteRef } from '@backstage/core';
+import { createRouteRef } from '@backstage/core-plugin-api';
 
 // Note: This route ref is for internal use only, don't export it from the plugin
 export const rootRouteRef = createRouteRef({
@@ -46,11 +46,11 @@ export const rootRouteRef = createRouteRef({
 
 Now that we have a `RouteRef`, we import it into `src/plugin.ts`, create our
 plugin instance with `createPlugin`, as well as create and wrap our routable
-extension using `createRoutableExtension` from `@backstage/core`:
+extension using `createRoutableExtension` from `@backstage/core-plugin-api`:
 
 ```tsx
 /* src/plugin.ts */
-import { createPlugin, createRouteRef } from '@backstage/core';
+import { createPlugin, createRouteRef } from '@backstage/core-plugin-api';
 import ExampleComponent from './components/ExampleComponent';
 
 // Create a plugin instance and export this from your plugin package

docs/plugins/structure-of-a-plugin.md

@@ -57,7 +57,10 @@ package.json to declare the plugin dependencies, metadata and scripts.
 In the `src` folder we get to the interesting bits. Check out the `plugin.ts`:
 
 ```jsx
-import { createPlugin, createRoutableExtension } from '@backstage/core';
+import {
+  createPlugin,
+  createRoutableExtension,
+} from '@backstage/core-plugin-api';
 
 import { rootRouteRef } from './routes';