Add Sass docs (variables, mixins, and loops) to most pages (#32747)

* WIP: Mention variables, mixins, and loops in docs

* Add Sass sections to component pages

* add sass docs for forms and content

* Update buttons.md

* Remove empty mixins sections

* Massive update to utilities and some consistency changes

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

scss/mixins/_list-group.scss

@@ -1,5 +1,6 @@
 // List Groups
 
+// scss-docs-start list-group-mixin
 @mixin list-group-item-variant($state, $background, $color) {
   .list-group-item-#{$state} {
     color: $color;
@@ -20,3 +21,4 @@
     }
   }
 }
+// scss-docs-end list-group-mixin