docs: add missing scss-docs-start comment

Author: mrholek

scss/mixins/_list-group.scss

@@ -1,5 +1,5 @@
 // List Groups
-
+// scss-docs-start list-group-mixin
 @mixin list-group-item-variant($state, $background, $background-hover, $color) {
   .list-group-item-#{$state} {
     --#{$variable-prefix}list-group-color: #{$color};