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/11-centering-lab/answer.css

@@ -0,0 +1,20 @@
+body {
+  margin: 0;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.stage {
+  min-height: 100vh;
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  background: linear-gradient(180deg, #fff7ed 0%, #fffbeb 100%);
+}
+
+.box {
+  padding: 28px 36px;
+  border-radius: 18px;
+  background: #ffffff;
+  border: 1px solid #fed7aa;
+  box-shadow: 0 20px 40px rgba(234, 88, 12, 0.12);
+}