docs: changing prod-url value in antora.yml

[max-cx]

What does this pull request change

Based on today's discussion with Fabrice and Ilya, changing the upstream value for the prod-url variable from https://che-host:che-port to https://__<che_fqdn>__.

What issues does this pull request fix or reference

Specify the version of the product this pull request applies to

Pull request checklist

The author and the reviewers validate the content of this pull request with the following checklist, in addition to the automated tests.

• Any procedure:
  • Successfully tested.
• Any page or link rename:
  • The page contains a redirection for the previous URL.
  • Propagate the URL change in:
    • Dashboard default branding data
    • Chectl constants.ts
• Builds on Eclipse Che hosted by Red Hat.
• the Validate language on files added or modified step reports no vale warnings.

[themr0c]

Outdated.