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/01-selectors-and-text/README.md

@@ -0,0 +1,32 @@
+# 练习 1：选择器与文本样式
+
+## 目标
+
+掌握最基础的 CSS 选择器和文字样式。
+
+## 你要练什么
+
+- 元素选择器
+- 类选择器
+- `color`
+- `font-size`
+- `font-weight`
+- `line-height`
+- `background-color`
+
+## 任务
+
+请把页面排成一个简洁的学习卡片，要求：
+
+- 页面背景换成浅色
+- 主标题颜色更明显
+- 介绍段落提高可读性
+- 给提示文字单独加一个背景色块
+- 给列表中的重点项单独强调
+
+## 文件
+
+- [starter.html](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/01-selectors-and-text/starter.html)
+- [starter.css](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/01-selectors-and-text/starter.css)
+- [answer.html](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/01-selectors-and-text/answer.html)
+- [answer.css](/Volumes/Macintosh HD 1/home/front-end-example/02-css-layout/01-selectors-and-text/answer.css)