[📖] Document expected environment variables

[ofavre]

Suggestion

It seems like some environment variables are pretty mandatory, at least as soon as we're not using a dev server anymore. See #3325 where @manucorporat suggests one should set the ORIGIN environment variable. I've just been bitten by this too.

Here is what Qwik City adapters use:

• Azure, Cloud Run, Netlify Edge, Node Server, use ORIGIN or URL.
• Cloudflare Pages uses CF_PAGES_URL or ORIGIN
• Vercel uses VERCEL_URL

Maybe this should be uniformized a bit more and deserves a unique documentation page. Or maybe each of these adapters should document the necessary environment variables on their respective documentation page.