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/_caret.scss

@@ -1,3 +1,4 @@
+// scss-docs-start caret-mixins
 @mixin caret-down {
   border-top: $caret-width solid;
   border-right: $caret-width solid transparent;
@@ -60,3 +61,4 @@
     }
   }
 }
+// scss-docs-end caret-mixins