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/_navbar.scss

@@ -154,6 +154,7 @@
   overflow-y: auto;
 }
 
+// scss-docs-start navbar-expand-loop
 // Generate series of `.navbar-expand-*` responsive classes for configuring
 // where your navbar collapses.
 .navbar-expand {
@@ -196,6 +197,7 @@
     }
   }
 }
+// scss-docs-end navbar-expand-loop
 
 
 // Navbar themes