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/10-fixed-and-sticky/answer.css

@@ -0,0 +1,35 @@
+body {
+  margin: 0;
+  background: #f8fafc;
+  font-family: "PingFang SC", "Microsoft YaHei", sans-serif;
+}
+
+.help {
+  position: fixed;
+  right: 20px;
+  bottom: 20px;
+  padding: 12px 16px;
+  border-radius: 999px;
+  background: #0f766e;
+  color: #ffffff;
+  text-decoration: none;
+}
+
+.page {
+  width: min(760px, calc(100% - 40px));
+  margin: 0 auto;
+}
+
+.topbar {
+  position: sticky;
+  top: 0;
+  padding: 16px;
+  background: #ffffff;
+  border-bottom: 1px solid #cbd5e1;
+  font-weight: 700;
+}
+
+.content {
+  min-height: 1200px;
+  padding: 24px 0 80px;
+}