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/_transitions.scss

@@ -6,6 +6,7 @@
   }
 }
 
+// scss-docs-start collapse-classes
 .collapse {
   &:not(.show) {
     display: none;
@@ -17,3 +18,4 @@
   overflow: hidden;
   @include transition($transition-collapse);
 }
+// scss-docs-end collapse-classes