rustdoc: do not animate when user prefers reduced motion

This accessibility improvement gates the target flashing and tooltip fade-out
behind an inverted prefers-reduced-motion media query.

Author: zopsicle

src/librustdoc/html/static/css/rustdoc.css

@@ -1687,7 +1687,12 @@ instead, we check that it's not a "finger" cursor.
 padding-right:3px;
 background-color:var(--target-background-color);
 border-right:3px solid var(--target-border-color);
-animation:0.65scubic-bezier(0,0,0.1,1.0) 0.1s targetfadein;
+}
+
+@medianot (prefers-reduced-motion) {
+:target {
+animation:0.65scubic-bezier(0,0,0.1,1.0) 0.1s targetfadein;
+	}
 }
 
 .code-headera.tooltip {
@@ -1712,12 +1717,14 @@ a.tooltip:hover::after {
 content:"\00a0";
 }
 
-/* This animation is layered onto the mistake-proofing delay for dismissing
-	a hovered tooltip, to ensure it feels responsive even with the delay.
-	*/
-.fade-out {
-opacity:0;
-transition: opacity 0.45scubic-bezier(0,0,0.1,1.0);
+@medianot (prefers-reduced-motion) {
+/* This animation is layered onto the mistake-proofing delay for dismissing
+ a hovered tooltip, to ensure it feels responsive even with the delay.
+ */
+	.fade-out {
+opacity:0;
+transition: opacity 0.45scubic-bezier(0,0,0.1,1.0);
+	}
 }
 
 .popover.tooltip .content {