docs: update plugin installation docs

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

plugins/cost-insights/README.md

@@ -51,11 +51,17 @@ export const apis = [
 ];
 ```
 
-4. Add cost-insights to your Backstage plugins.
+4. Add the `CostInsightsPage` extension to your `App.tsx`:
 
-```ts
-// packages/app/src/plugins.ts
-export { plugin as CostInsights } from '@backstage/plugin-cost-insights';
+```tsx
+// packages/app/src/App.tsx
+import { CostInsightsPage } from '@backstage/plugin-cost-insights';
+
+<FlatRoutes>
+  ...
+  <Route path="/cost-insights" element={<CostInsightsPage />} />
+  ...
+</FlatRoutes>;
 ```
 
 5. Add Cost Insights to your app Sidebar.