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/_alert.scss

@@ -1,3 +1,4 @@
+// scss-docs-start alert-variant-mixin
 @mixin alert-variant($background, $border, $color) {
   color: $color;
   @include gradient-bg($background);
@@ -7,3 +8,4 @@
     color: shade-color($color, 20%);
   }
 }
+// scss-docs-end alert-variant-mixin