add redux intro (#15605)

seed/challenges/03-front-end-libraries/redux.json

@@ -8,6 +8,12 @@
       "id": "587d7dbd367417b2b2512bb2",
       "title": "Introduction to the Redux Challenges",
       "description": [
+        [
+          "",
+          "",
+          "Redux is self-described as a \"predictable state container for JavaScript apps.\" State (as introduced in the React challenges) is a way to manage what is displayed to a user based on that user's inputs or interactions with the application. While it's not necessary to use Redux with React, or vice-versa, the two are often used in conjunction as the features of Redux support applications built with React. Some of these features include:<br><br><ol><li>An immutable single state store or \"single state of truth\" through which the application makes UI updates</li><li>Logical structure of `actions` carrying data to `reducer` functions that return a new application state</li><li>A trackable history of changes to state</li></ol><br><br>Redux is often added to a project when the complexity of managing the application's state becomes tedious. It helps minimize some of the headaches that can be caused by data mutability or passing state back-and-forth between many React components. Redux also supports debugging with tools like <a href=\"https://github.com/gaearon/redux-devtools\" target=\"_blank\">Redux DevTools</a>. Let's jump in to learning how Redux can simplify your React apps.",
+          ""
+        ],
         [
           "",
           "",