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.

02-css-layout/03-flex-layout/README.md

@@ -0,0 +1,30 @@
+# 练习 3：Flex 布局
+
+## 目标
+
+学会用 Flex 做横向排列、间距和居中。
+
+## 你要练什么
+
+- `display: flex`
+- `justify-content`
+- `align-items`
+- `gap`
+- `flex: 1`
+
+## 任务
+
+请完成一个三列学习面板，要求：
+
+- 3 个卡片横向排列
+- 卡片之间有统一间距
+- 每个卡片宽度尽量均分
+- 标题和按钮排版清晰
+- 整个区域居中显示
+
+## 文件
+
+- [starter.html](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/03-flex-layout/starter.html)
+- [starter.css](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/03-flex-layout/starter.css)
+- [answer.html](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/03-flex-layout/answer.html)
+- [answer.css](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/03-flex-layout/answer.css)