fix(docs-site): rename hero-bg.jpg to .png (file was PNG with wrong extension)

The hero background image was a PNG file incorrectly named with .jpg extension.
GitHub Pages was serving it with content-type: image/jpeg, causing browsers
to reject rendering due to MIME type mismatch.

Author: snowarch

docs-site/public/hero-bg.jpg docs-site/public/hero-bg.pngdocs-site/public/hero-bg.jpg renamed to docs-site/public/hero-bg.png

@@ -179,7 +179,7 @@ export default function HomePage() {
       <div className="relative overflow-hidden">
         <div className="absolute inset-0 z-0">
           <img
-            src={`${process.env.NEXT_PUBLIC_BASE_PATH || ""}/hero-bg.jpg`}
+            src={`${process.env.NEXT_PUBLIC_BASE_PATH || ""}/hero-bg.png`}
             alt=""
             className="h-full w-full object-cover opacity-30"
             aria-hidden="true"