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/02-box-model-card/README.md

@@ -0,0 +1,31 @@
+# 练习 2：盒模型卡片
+
+## 目标
+
+彻底理解 `margin`、`padding`、`border` 的区别。
+
+## 你要练什么
+
+- `width`
+- `padding`
+- `border`
+- `margin`
+- `border-radius`
+- `box-shadow`
+
+## 任务
+
+请把一个课程卡片做出来，要求：
+
+- 卡片和页面边缘有距离
+- 卡片内部文字和边框有距离
+- 卡片有边框和圆角
+- 卡片有轻微阴影
+- 按钮区域和正文之间有明显间隔
+
+## 文件
+
+- [starter.html](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/02-box-model-card/starter.html)
+- [starter.css](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/02-box-model-card/starter.css)
+- [answer.html](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/02-box-model-card/answer.html)
+- [answer.css](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/02-box-model-card/answer.css)