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/07-selectors-and-pseudo-classes/answer.css

@@ -0,0 +1,23 @@
+body {
+  margin: 40px;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.menu a {
+  color: #475569;
+  text-decoration: none;
+}
+
+.menu > a {
+  margin-right: 16px;
+  font-weight: 600;
+}
+
+.menu a:hover {
+  color: #2563eb;
+}
+
+.list li:first-child {
+  color: #b45309;
+  font-weight: 700;
+}