Gradients and shadows (#24429)

* Update the form focus mixin to use a manual `$enable-shadows` check so we can always ensure a focus state for accessibility and consistency

* - Add new `$input-btn-focus-width` and `$input-btn-focus-color` variables.

- Replace separate `$btn-focus-box-shadow` and `$input-focus-box-shadow`
variables with unified `$input-btn-focus-box-shadow` to match our
combined variables approach elsewhere.

* Put new focus width var to use in buttons mixins

* use new button input-box shadow var

* Add a new mixin for quickly adding linear gradient to components when $enable-gradients is set to true

* use correct var

* fix focus shadows in button mixins

* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items

* Generate .bg-gradient- utilities

* add headings to colors page and document bg-gradient utils

* update the button color for active status, check with yiq as it's done for basic state and hover state

scss/utilities/_background.scss

@@ -4,6 +4,12 @@
   @include bg-variant(".bg-#{$color}", $value);
 }
 
+@if $enable-gradients {
+  @each $color, $value in $theme-colors {
+    @include bg-gradient-variant(".bg-gradient-#{$color}", $value);
+  }
+}
+
 .bg-white {
   background-color: $white !important;
 }