doc: add StateProvider and StateValue

src/redux/StateProvider.js

@@ -0,0 +1,11 @@
+import React, { createContext, useContext, useReducer } from "react";
+
+export const StateContext = createContext();
+
+export const StateProvider = ({ reducer, initialState, children }) => (
+  <StateContext.Provider value={useReducer(reducer, initialState)}>
+    {children}
+  </StateContext.Provider>
+);
+
+export const useStateValue = () => useContext(StateContext);