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/answer.css

@@ -0,0 +1,31 @@
+body {
+  margin: 0;
+  min-height: 100vh;
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  background: #f8fafc;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.panel {
+  width: min(1100px, calc(100% - 48px));
+  display: flex;
+  gap: 20px;
+}
+
+.item {
+  flex: 1;
+  padding: 24px;
+  border-radius: 18px;
+  background: #ffffff;
+  border: 1px solid #e2e8f0;
+}
+
+a {
+  display: inline-block;
+  margin-top: 16px;
+  color: #0f766e;
+  text-decoration: none;
+  font-weight: 700;
+}