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/answer.css

@@ -0,0 +1,37 @@
+body {
+  margin: 0;
+  background: #eef2ff;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.course-card {
+  width: 360px;
+  margin: 56px auto;
+  padding: 28px;
+  border: 1px solid #c7d2fe;
+  border-radius: 20px;
+  background: #ffffff;
+  box-shadow: 0 16px 40px rgba(37, 99, 235, 0.12);
+}
+
+.tag {
+  display: inline-block;
+  margin: 0;
+  padding: 6px 12px;
+  border-radius: 999px;
+  background: #dbeafe;
+  color: #1d4ed8;
+}
+
+.actions {
+  margin-top: 24px;
+}
+
+a {
+  display: inline-block;
+  padding: 10px 18px;
+  border-radius: 10px;
+  background: #1d4ed8;
+  color: #ffffff;
+  text-decoration: none;
+}