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/_progress.scss

@@ -1,9 +1,12 @@
 // Disable animation if transitions are disabled
+
+// scss-docs-start progress-keyframes
 @if $enable-transitions {
   @keyframes progress-bar-stripes {
     0% { background-position-x: $progress-height; }
   }
 }
+// scss-docs-end progress-keyframes
 
 .progress {
   display: flex;