Slow down spinners when prefers-reduced-motion: reduce (#31882)

* feature(spinners): slow down spinners when prefers-reduced-motion

* docs(spinners): add reduced motion callout and mention slowing down in accessibility page

* Update spinners.md

* docs(accessibility): rewording

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

scss/_spinners.scss

@@ -54,3 +54,12 @@
   width: $spinner-width-sm;
   height: $spinner-height-sm;
 }
+
+@if $enable-reduced-motion {
+  @media (prefers-reduced-motion: reduce) {
+    .spinner-border,
+    .spinner-grow {
+      animation-duration: $spinner-animation-speed * 2;
+    }
+  }
+}