chore: updated icon for an exercise box in introduction page

- Modified introduction.js to update the exercise box icon.
- Added exercise2.png as the new icon in the  directory.

Author: SlayeR6889

src/components/Docs/introduction.js

@@ -118,7 +118,7 @@ const IntroductionComponent = () => {
         <Link to={`/develop/exercise`} className="docs_card_with_image">
           <div>
             <img
-              src="/img/exercise.png"
+              src="/img/exercise2.png"
               alt="airchains"
               className="junction_logo"
             />