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/08-overflow-and-sizing/answer.css

@@ -0,0 +1,17 @@
+body {
+  margin: 0;
+  background: #f1f5f9;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.panel {
+  width: 320px;
+  height: 180px;
+  margin: 48px auto;
+  padding: 18px;
+  overflow: auto;
+  background: #ffffff;
+  border: 1px solid #cbd5e1;
+  border-radius: 16px;
+  box-sizing: border-box;
+}