chore(shell-docs): pre-bake WebP for gen-ui-specs diagrams

Wave-2 follow-up to CopilotKit#4986. With the next/image optimizer disabled
(images.unoptimized: true), gen-ui-specs-light.png and gen-ui-specs-dark.png
were shipping uncompressed instead of being served as WebP via /_next/image.
Pre-baking WebP variants restores the bandwidth savings: light 408,807 -> 126,982
bytes (3.2x smaller) and dark 496,127 -> 167,118 bytes (3.0x smaller). Source
PNGs are retained alongside as fallbacks.

Author: jpr5

showcase/shell-docs/public/images/gen-ui-specs-dark.webp

@@ -1,14 +1,14 @@
 import Image from "next/image";
 
 <Image
-  src="/images/gen-ui-specs-light.png"
+  src="/images/gen-ui-specs-light.webp"
   alt="Generative UI Specs"
   width={4096}
   height={2304}
   className="block dark:hidden mb-8"
 />
 <Image
-  src="/images/gen-ui-specs-dark.png"
+  src="/images/gen-ui-specs-dark.webp"
   alt="Generative UI Specs"
   width={4096}
   height={2304}

showcase/shell-docs/public/images/gen-ui-specs-light.webp

@@ -5,14 +5,14 @@ import Image from "next/image";
 **A2UI** is Google's declarative, LLM-friendly Generative UI specification that enables agents to generate dynamic user interfaces.
 
 <Image
-  src="/images/gen-ui-specs-light.png"
+  src="/images/gen-ui-specs-light.webp"
   alt="Generative UI Specs"
   width={4096}
   height={2304}
   className="block dark:hidden mb-8"
 />
 <Image
-  src="/images/gen-ui-specs-dark.png"
+  src="/images/gen-ui-specs-dark.webp"
   alt="Generative UI Specs"
   width={4096}
   height={2304}