Upload nightly and release versions from CI

[Indy2222]

• Use GitHub pages
  • Repository: https://github.com/DigitalExtinction/Downloads
  • Domain: download.de-game.org
• Push these files (from this repo to the GH Pages repo):
  • nightly.zip
• Document & link these files from README.md
• Link these files from de-game.org

Relates to #272 and #529.