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/forms/_form-select.scss

@@ -0,0 +1,76 @@
+// Select
+//
+// Replaces the browser default select with a custom one, mostly pulled from
+// https://primer.github.io/.
+
+.form-select {
+  display: inline-block;
+  width: 100%;
+  height: $form-select-height;
+  padding: $form-select-padding-y ($form-select-padding-x + $form-select-indicator-padding) $form-select-padding-y $form-select-padding-x;
+  font-family: $form-select-font-family;
+  @include font-size($form-select-font-size);
+  font-weight: $form-select-font-weight;
+  line-height: $form-select-line-height;
+  color: $form-select-color;
+  vertical-align: middle;
+  background: $form-select-background;
+  background-color: $form-select-bg;
+  border: $form-select-border-width solid $form-select-border-color;
+  @include border-radius($form-select-border-radius, 0);
+  @include box-shadow($form-select-box-shadow);
+  appearance: none;
+
+  &:focus {
+    border-color: $form-select-focus-border-color;
+    outline: 0;
+    @if $enable-shadows {
+      box-shadow: $form-select-box-shadow, $form-select-focus-box-shadow;
+    } @else {
+      box-shadow: $form-select-focus-box-shadow;
+    }
+
+    &::-ms-value {
+      // For visual consistency with other platforms/browsers,
+      // suppress the default white text on blue background highlight given to
+      // the selected option text when the (still closed) <select> receives focus
+      // in IE and (under certain conditions) Edge.
+      // See https://github.com/twbs/bootstrap/issues/19398.
+      color: $input-color;
+      background-color: $input-bg;
+    }
+  }
+
+  &[multiple],
+  &[size]:not([size="1"]) {
+    height: auto;
+    padding-right: $form-select-padding-x;
+    background-image: none;
+  }
+
+  &:disabled {
+    color: $form-select-disabled-color;
+    background-color: $form-select-disabled-bg;
+  }
+
+  // Hides the default caret in IE11
+  &::-ms-expand {
+    display: none;
+  }
+}
+
+.form-select-sm {
+  height: $form-select-height-sm;
+  padding-top: $form-select-padding-y-sm;
+  padding-bottom: $form-select-padding-y-sm;
+  padding-left: $form-select-padding-x-sm;
+  @include font-size($form-select-font-size-sm);
+}
+
+.form-select-lg {
+  height: $form-select-height-lg;
+  padding-top: $form-select-padding-y-lg;
+  padding-bottom: $form-select-padding-y-lg;
+  padding-left: $form-select-padding-x-lg;
+  @include font-size($form-select-font-size-lg);
+}