Add an image optimization example to docs

[MarkBennett]

Serving optimally sized images on-demand and without a build step is an important factor in SEO and a barrier for devs looking to try Fresh on their sites.

It would be great to collect some best practices and share some examples of serving optimized images with Fresh in the docs.

Happy to do a PR if this is something you're interested in @lucacasonato.

[bartlomieju]

Fresh 2 now uses Vite, so standard Vite image optimization plugins (like vite-imagetools or @sveltejs/enhanced-img) can be used directly. The docs cover asset() and assetSrcSet() for cache-busted image URLs with responsive srcset support. Image optimization itself is outside Fresh's scope — it's best handled by Vite plugins, CDN image services (Cloudflare Images, imgix, etc.), or build-time tools.