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/_buttons.scss

@@ -55,6 +55,7 @@
 // Alternate buttons
 //
 
+// scss-docs-start btn-variant-loops
 @each $color, $value in $theme-colors {
   .btn-#{$color} {
     @include button-variant($value, $value);
@@ -66,6 +67,7 @@
     @include button-outline-variant($value);
   }
 }
+// scss-docs-end btn-variant-loops
 
 
 //