docs: address copilot review comments

Add installation example for extraIcons in the customize-theme guide
and fix "productional code" typo in quickstart-app-plugin--old.

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

docs/golden-path/create-app/customize-theme.md

@@ -466,6 +466,8 @@ export default app.createRoot();
 You can register additional icons so that they can be used in other places like entity links. For example, to add an `alert` icon:
 
 ```tsx title="packages/app/src/App.tsx"
+import { createApp } from '@backstage/frontend-defaults';
+import { createFrontendModule } from '@backstage/frontend-plugin-api';
 import AlarmIcon from '@material-ui/icons/Alarm';
 import { IconBundleBlueprint } from '@backstage/plugin-app-react';
 
@@ -477,6 +479,17 @@ const extraIcons = IconBundleBlueprint.make({
     },
   },
 });
+
+const app = createApp({
+  features: [
+    createFrontendModule({
+      pluginId: 'app',
+      extensions: [extraIcons],
+    }),
+  ],
+});
+
+export default app.createRoot();
 ```
 
 You can then reference `alert` for your icon in entity links like this:

docs/tutorials/quickstart-app-plugin--old.md

@@ -155,7 +155,7 @@ export const ExampleFetchComponent = () => {
 3. Save that and ensure you see no errors. Comment out the unused imports if
    your linter gets in the way.
 
-###### We will add a lot to this file for the sake of ease. Please don't do this in productional code!
+###### We will add a lot to this file for the sake of ease. Please don't do this in production code!
 
 ## The Graph Model