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/_alert.scss

@@ -1,3 +1,4 @@
+// scss-docs-start alert-variant-mixin
 @mixin alert-variant($background, $border, $color) {
   color: $color;
   @include gradient-bg($background);
@@ -7,3 +8,4 @@
     color: shade-color($color, 20%);
   }
 }
+// scss-docs-end alert-variant-mixin

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

scss/mixins/_buttons.scss

@@ -3,6 +3,7 @@
 // Easily pump out default styles, as well as :hover, :focus, :active,
 // and disabled options for all buttons
 
+// scss-docs-start btn-variant-mixin
 @mixin button-variant(
   $background,
   $border,
@@ -71,7 +72,9 @@
     border-color: $disabled-border;
   }
 }
+// scss-docs-end btn-variant-mixin
 
+// scss-docs-start btn-outline-variant-mixin
 @mixin button-outline-variant(
   $color,
   $color-hover: color-contrast($color),
@@ -118,11 +121,13 @@
     background-color: transparent;
   }
 }
+// scss-docs-end btn-outline-variant-mixin
 
-// Button sizes
+// scss-docs-start btn-size-mixin
 @mixin button-size($padding-y, $padding-x, $font-size, $border-radius) {
   padding: $padding-y $padding-x;
   @include font-size($font-size);
   // Manually declare to provide an override to the browser default
   @include border-radius($border-radius, 0);
 }
+// scss-docs-end btn-size-mixin

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

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

scss/mixins/_list-group.scss

@@ -1,5 +1,6 @@
 // List Groups
 
+// scss-docs-start list-group-mixin
 @mixin list-group-item-variant($state, $background, $color) {
   .list-group-item-#{$state} {
     color: $color;
@@ -20,3 +21,4 @@
     }
   }
 }
+// scss-docs-end list-group-mixin

scss/mixins/_pagination.scss

@@ -1,5 +1,6 @@
 // Pagination
 
+// scss-docs-start pagination-mixin
 @mixin pagination-size($padding-y, $padding-x, $font-size, $border-radius) {
   .page-link {
     padding: $padding-y $padding-x;
@@ -27,3 +28,4 @@
     }
   }
 }
+// scss-docs-end pagination-mixin