Docs: update Webpack guide (#38345)

* Docs: remove `<script>` from `index.html` in Webpack guide

* Add `mode=production` to `build` step in Webpack guide

site/content/docs/5.3/getting-started/webpack.md

@@ -115,7 +115,6 @@ With dependencies installed and our project folder ready for us to start coding,
          <h1>Hello, Bootstrap and Webpack!</h1>
          <button class="btn btn-primary">Primary button</button>
        </div>
-       <script src="./main.js"></script>
      </body>
    </html>
    ```
@@ -129,7 +128,7 @@ With dependencies installed and our project folder ready for us to start coding,
      // ...
      "scripts": {
        "start": "webpack serve",
-       "build": "webpack build",
+       "build": "webpack build --mode=production",
        "test": "echo \"Error: no test specified\" && exit 1"
      },
      // ...