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/03-semantic-layout/README.md

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+# 练习 3：语义化布局
+
+## 目标
+
+学会把一个页面拆成有意义的结构区域。
+
+## 你要练什么
+
+- `header`
+- `nav`
+- `main`
+- `section`
+- `article`
+- `aside`
+- `footer`
+
+## 任务
+
+请完成一个“个人博客首页结构”，要求包含：
+
+- 页头：博客名称
+- 导航：至少 3 个链接
+- 主内容区
+- 一个“最新文章”分区
+- 分区里至少 2 篇 `article`
+- 一个侧边栏，写上“关于我”
+- 一个页脚
+
+## 检查点
+
+- 页面是否有清晰的区域划分
+- `article` 是否适合承载独立内容
+- `aside` 是否作为补充信息
+- `footer` 是否放在页面结尾
+
+## 文件
+
+- [starter.html](/Volumes/Macintosh HD 1/home/front-end-example/01-html-structure/03-semantic-layout/starter.html)
+- [answer.html](/Volumes/Macintosh HD 1/home/front-end-example/01-html-structure/03-semantic-layout/answer.html)