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/_border-radius.scss

@@ -14,6 +14,7 @@
   @return $return;
 }
 
+// scss-docs-start border-radius-mixins
 @mixin border-radius($radius: $border-radius, $fallback-border-radius: false) {
   @if $enable-rounded {
     border-radius: valid-radius($radius);
@@ -74,3 +75,4 @@
     border-bottom-left-radius: valid-radius($radius);
   }
 }
+// scss-docs-end border-radius-mixins