doc: add StateProvider, reducer, and initialState to entry file

src/index.js

@@ -2,10 +2,14 @@ import React from "react";
 import ReactDOM from "react-dom";
 import "./index.css";
 import App from "./App";
+import { StateProvider } from "./redux/StateProvider";
+import reducer, { initialState } from "./redux/reducer";
 
 ReactDOM.render(
   <React.StrictMode>
-    <App />
+    <StateProvider initialState={initialState} reducer={reducer}>
+      <App />
+    </StateProvider>
   </React.StrictMode>,
   document.getElementById("root")
 );