v5: Forms update (#28450)

* Initial spike of consolidated form checks

* Stub out forms rearrangement

- Prepping to drop non-custom file and range inputs
- Prepping to merge custom and native checks and radios (with switches)
- Prepping to merge custom select with form select
- Moving docs arround so forms has it's own area given volume of CSS

* Move input group Sass file to forms subdir

* Start to split and move the docs around

* Simpler imports

* Copyediting

* delete overview file

* Remove commented out code

* remove the custom-forms import

* rewrite flex-check as form-check, replace all custom properties

* Remove old forms doc

* stub out new subpage link section

* update migration guide

* Update nav, forms overview in page nav, and descriptions

* fix check bg position

* fix margin-top calculation

* rename .custom-select to .form-select

* Update validation styles for new checks

* add some vertical margin, fix inline checks

* fix docs examples

* better way to do this contents stuff, redo the toc while i'm at it

* page restyle for docs while here

* un-callout that, edit text

* redo padding on toc

* fix toc

* start to cleanup checks docs

* Rewrite Markdown tables into HTML

* Redesign tables, redo their docs

* Replace Open Iconic icons with custom Bootstrap icons

* Redesign the docs navbar, add a subheader, redo the sidebar

* Redesign docs homepage a bit

* Simplify table style overrides for docs tables

* Simplify docs typography for page titles and reading line length

* Stub out icons page

* Part of sidebar update, remove migration from nav.yml

* Move toc CSS to separate partial

* Change appearance of overview page

* fix sidebar arrow direction

* Add footer to docs layout

* Update descriptions

* Drop the .form-group class for margin utilities

* Remove lingering form-group-margin-bottom var

* improve footer spacing

* add headings to range page

* uncomment form range css

* Rename .custom-range to .form-range

* Drop unused docs var

* Uncomment the comment

* Remove unused variable

* Fix radio image sizing

* Reboot update: reset horizontal ul and ol padding

* de-dupe IDs

* tweak toc styles

* nvm, fix dropdown versions stuff

* remove sidebar nav toggle for now

* broken html

* fix more broken html, move css

* scss linting

* comment out broken helper docs

* scope styles

* scope styles

* Fixes #25540 and fixes #26407 for v5 only

* Update sidebar once more

* Match new sidenav order

* fix syntax error

* Rename custom-file to form-file, update paths, update migration docs for previous changes in #28696

* rename back

* fix size and alignment

* rename that back too

scss/mixins/_forms.scss

@@ -97,13 +97,13 @@
     }
   }
 
-  .custom-select {
+  .form-select {
     @include form-validation-state-selector($state) {
       border-color: $color;
 
       @if $enable-validation-icons {
-        padding-right: $custom-select-feedback-icon-padding-right;
-        background: $custom-select-background, $icon $custom-select-bg no-repeat $custom-select-feedback-icon-position / $custom-select-feedback-icon-size;
+        padding-right: $form-select-feedback-icon-padding-right;
+        background: $form-select-background, $icon $form-select-bg no-repeat $form-select-feedback-icon-position / $form-select-feedback-icon-size;
       }
 
       &:focus {
@@ -130,6 +130,16 @@
 
   .form-check-input {
     @include form-validation-state-selector($state) {
+      border-color: $color;
+
+      &:checked {
+        @include gradient-bg(lighten($color, 10%));
+      }
+
+      &:focus {
+        box-shadow: 0 0 0 $input-focus-width rgba($color, .25);
+      }
+
       ~ .form-check-label {
         color: $color;
       }
@@ -140,45 +150,16 @@
       }
     }
   }
-
-  .custom-control-input {
-    @include form-validation-state-selector($state) {
-      ~ .custom-control-label {
-        color: $color;
-
-        &::before {
-          border-color: $color;
-        }
-      }
-
-      ~ .#{$state}-feedback,
-      ~ .#{$state}-tooltip {
-        display: block;
-      }
-
-      &:checked {
-        ~ .custom-control-label::before {
-          border-color: lighten($color, 10%);
-          @include gradient-bg(lighten($color, 10%));
-        }
-      }
-
-      &:focus {
-        ~ .custom-control-label::before {
-          box-shadow: 0 0 0 $input-focus-width rgba($color, .25);
-        }
-
-        &:not(:checked) ~ .custom-control-label::before {
-          border-color: $color;
-        }
-      }
+  .form-check-inline .form-check-input {
+    ~ .#{$state}-feedback {
+      margin-left: .5em;
     }
   }
 
   // custom file
-  .custom-file-input {
+  .form-file-input {
     @include form-validation-state-selector($state) {
-      ~ .custom-file-label {
+      ~ .form-file-label {
         border-color: $color;
       }
 
@@ -188,7 +169,7 @@
       }
 
       &:focus {
-        ~ .custom-file-label {
+        ~ .form-file-label {
           border-color: $color;
           box-shadow: 0 0 0 $input-focus-width rgba($color, .25);
         }