Add support for Pages and Sidebar items in existing app

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Camila Belo <camilaibs@gmail.com>
Co-authored-by: Philipp Hugenroth <philipph@spotify.com>
Signed-off-by: Johan Haals <johan.haals@gmail.com>

packages/app/package.json

@@ -20,6 +20,7 @@
     "@backstage/core-app-api": "workspace:^",
     "@backstage/core-components": "workspace:^",
     "@backstage/core-plugin-api": "workspace:^",
+    "@backstage/frontend-app-api": "workspace:^",
     "@backstage/integration-react": "workspace:^",
     "@backstage/plugin-adr": "workspace:^",
     "@backstage/plugin-airbrake": "workspace:^",