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/_forms.scss

@@ -1,5 +1,7 @@
 // This mixin uses an `if()` technique to be compatible with Dart Sass
 // See https://github.com/sass/sass/issues/1873#issuecomment-152293725 for more details
+
+// scss-docs-start form-validation-mixins
 @mixin form-validation-state-selector($state) {
   @if ($state == "valid" or $state == "invalid") {
     .was-validated #{if(&, "&", "")}:#{$state},
@@ -122,3 +124,4 @@
     }
   }
 }
+// scss-docs-end form-validation-mixins