docs(cloudflare): add static assets caching

[benjavicente]

Solves opennextjs/opennextjs-cloudflare#624 (comment)

[benjavicente]

Opened a PR on the starter template: cloudflare/workers-sdk#9131

[benjavicente]

The closest docs about Next.js's immutable cache header is on the Next.js 13 Pages Router production checklist:

Caching improves response times and reduces the number of requests to external services. Next.js automatically adds caching headers to immutable assets served from /_next/static including JavaScript, CSS, static images, and other media.

Cache-Control: public, max-age=31536000, immutable

On the Next.js 15 App Router docs, only the header is mentioned. I'm trying to confirm that ALL files inside /_next/static are immutable in modern versions of Next.js, but with the old documentation and that I couldn't find a production Next.js site without a file in on /_next/static/* that didn't have a hash on its path or it's missing that header, I believe it's ok to add this default.

[vicb]

Thanks for the PR Benjamin, I'll merge when the test result comes.