rustdoc: do not animate :target when user prefers reduced motion

This gates rust-lang#129284 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: 0.65s cubic-bezier(0, 0, 0.1, 1.0) 0.1s targetfadein;
+}
+
+@media not (prefers-reduced-motion) {
+	:target {
+		animation: 0.65s cubic-bezier(0, 0, 0.1, 1.0) 0.1s targetfadein;
+	}
 }
 
 .code-header a.tooltip {