style: update header style on the demo page

Author: julhoang

static/css/v3/v3-examples-section.css

@@ -6,13 +6,6 @@
   color: var(--color-text-primary);
 }
 
-body:has(.v3-examples-section) header {
-  position: absolute;
-  z-index: 10;
-  left: 0;
-  right: 0;
-}
-
 .v3-examples-section__heading {
   font-size: var(--font-size-xl, 20px);
   font-weight: var(--font-weight-medium, 500);