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/01-selectors-and-text/answer.css

@@ -0,0 +1,37 @@
+body {
+  margin: 0;
+  background-color: #f5f7fb;
+  color: #1f2937;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.card {
+  width: 720px;
+  margin: 48px auto;
+  padding: 24px;
+  background-color: #ffffff;
+  border: 1px solid #dbe3ee;
+  border-radius: 16px;
+}
+
+h1 {
+  color: #1d4ed8;
+  font-size: 32px;
+}
+
+.intro {
+  font-size: 16px;
+  line-height: 1.8;
+}
+
+.note {
+  background-color: #e0f2fe;
+  color: #0f172a;
+  padding: 12px;
+  border-radius: 10px;
+}
+
+.important {
+  color: #b45309;
+  font-weight: 700;
+}