Revert "Replace GitHub Button"

This reverts commit 1844b86.

static/frontpage/frontpage.html

@@ -204,7 +204,7 @@ <h2 class="mdl-color-text--primary">《动手学深度学习》</h2>
     <p>面向中文读者的能运行、可讨论的深度学习教科书</p>
     <p>含 NumPy/MXNet、PyTorch 和 TensorFlow 实现</p>
     <p>被全球 60 个国家 400 所大学用于教学</p>
-    <p><iframe src="https://ghbtns.com/github-btn.html?user=d2l-ai&repo=d2l-zh&type=star&count=true&size=large" frameborder="0" scrolling="0" width="170" height="30" title="GitHub"></iframe></p>
+    <p><a class="github-button" href="https://github.com/d2l-ai/d2l-zh" data-icon="octicon-star" data-size="large" data-show-count="true" aria-label="Star d2l-ai/d2l-zh on GitHub">Star</a></p>
   </div>
 </div>
 
@@ -1247,4 +1247,5 @@ <h2 class="toc">目录</h2>
 }
 </script>
 
+<script async defer src="https://buttons.github.io/buttons.js"></script>