PLANET-5729 Add content-visibility to lazy load below the fold elements (#2804)

This commit adds it to the footer and the Related Posts

Author: mleray

assets/src/scss/layout/_footer.scss

@@ -5,6 +5,7 @@
     color: var(--white);
     line-height: 0;
   }
+  content-visibility: auto;
   position: relative;
   z-index: 2;
   text-align: center;

assets/src/scss/pages/post/_post.scss

@@ -84,6 +84,7 @@
 }
 
 .post-articles-block {
+  content-visibility: auto;
   margin-top: $sp-4;
 
   .posts-list {