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/_modal.scss

@@ -202,6 +202,7 @@
   .modal-xl { max-width: $modal-xl; }
 }
 
+// scss-docs-start modal-fullscreen-loop
 @each $breakpoint in map-keys($grid-breakpoints) {
   $infix: breakpoint-infix($breakpoint, $grid-breakpoints);
   $postfix: if($infix != "", $infix + "-down", "");
@@ -233,3 +234,4 @@
     }
   }
 }
+// scss-docs-end modal-fullscreen-loop