feat(css): vertically align <section> contents

Author: coolaj86

slides.css

@@ -125,6 +125,7 @@ section {
   aspect-ratio: 16 / 9;
   display: flex;
   flex-direction: column;
+  justify-content: center;
   gap: 2.5cqw;
   border-color: #0000ff0f;
   background-color: #f0f8ff;