add reducer example to recipes sidebar

Author: alexkrolick

website/sidebars.json

@@ -97,6 +97,7 @@
  "example-input-event",
  "example-update-props",
  "example-react-context",
+ "example-react-hooks-useReducer",
  "example-react-intl",
  "example-react-redux",
  "example-react-router",