Update next.js documentation example

What does this MR do?

The npm run build command is defined in the package.json as next build && next export. Since the create-next-app includes a dynamic route I've removed it as well as disabled optimized images since they do not work with static files:

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: true,
  images: {
    unoptimized: true,
  },
  assetPrefix: "https://gitlab-gold.gitlab.io/faleksic-tests/public-group/gitlab-pages-testing/"

}
module.exports = nextConfig

We also need to include an assetPrefix to make it work. From the NextJS documentation:

While assetPrefix covers requests to _next/static, it does not influence the following paths:
- Files in the public folder; if you want to serve those assets over a CDN, you'll have to introduce the prefix yourself

Working example can be found here - https://gitlab-gold.gitlab.io/faleksic-tests/public-group/gitlab-pages-testing/

With the code behind it in - https://gitlab.com/gitlab-gold/faleksic-tests/public-group/gitlab-pages-testing

Related issues

#404603 (closed)

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