Rename the classes and docs page, update everything accordingly

site/assets/scss/_component-examples.scss

@@ -166,13 +166,13 @@
   }
 
   // Responsive embed helpers
-  .embed-responsive {
+  .ratio {
     display: inline-block;
     color: $gray-600;
     background-color: $gray-100;
     border: $border-width solid $border-color;
   }
-  .embed-responsive-item {
+  .ratio-item {
     display: flex;
     align-items: center;
     justify-content: center;