[Speed] Mirage deprecation notice

Author: ToriLindsay

src/content/docs/speed/optimization/images/mirage.mdx

@@ -12,6 +12,10 @@ sidebar:
 
 import { FeatureTable, GlossaryTooltip, Render } from "~/components"
 
+:::caution[Deprecation notice]
+Mirage is being deprecated and will be removed on XXXX-XX-XX. Cloudflare recommends using [lazy loading](/images/tutorials/optimize-mobile-viewing/#lazy-loading) as an alternative to Mirage to optimize the images on your webpages for mobile viewing.
+:::
+
 Cloudflare Mirage reduces bandwidth used by images in mobile browsers. It can accelerate loading of image-heavy websites on very slow mobile connections and HTTP/1.
 
 It does this by: