feat: Add CSS layout exercises and corresponding HTML files

- Created multiple exercises under the CSS layout section, including:
  - Final page layout with CSS styles and HTML structure.
  - Display and flow concepts with examples of block, inline, and none display types.
  - Selectors and pseudo-classes with practical examples.
  - Overflow and sizing handling in CSS.
  - Grid layout basics for two-dimensional layouts.
  - Fixed and sticky positioning examples.
  - Centering techniques for common layout scenarios.

- Added README files for each exercise to outline objectives and file structures.
- Updated main README to include new sections and usage instructions.

01-html-structure/09-nesting-review/README.md

@@ -0,0 +1,21 @@
+# 练习 9：嵌套纠错
+
+## 目标
+
+训练你检查 HTML 结构是否写错。
+
+## 你要练什么
+
+- 标签闭合
+- 父子关系
+- 列表嵌套
+- 表单结构
+
+## 任务
+
+先看 `starter.html`，里面故意写了一些常见错误。你的任务是自己修正，再对照 `answer.html`。
+
+## 文件
+
+- [starter.html](/Volumes/Macintosh HD 1/home/front-end-example/01-html-structure/09-nesting-review/starter.html)
+- [answer.html](/Volumes/Macintosh HD 1/home/front-end-example/01-html-structure/09-nesting-review/answer.html)