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/06-display-and-flow/answer.css

@@ -0,0 +1,29 @@
+body {
+  margin: 0;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+  background: #f8fafc;
+}
+
+.wrap {
+  width: 760px;
+  margin: 40px auto;
+}
+
+.tag {
+  display: inline-block;
+  padding: 8px 14px;
+  margin-right: 10px;
+  background: #dbeafe;
+  border-radius: 999px;
+}
+
+.hide-me {
+  display: none;
+}
+
+.box {
+  margin-top: 16px;
+  padding: 16px;
+  background: #ffffff;
+  border: 1px solid #cbd5e1;
+}