Improve structured logging and validation

[bernhardreiter]

As followup to #443, here are some minor issues and questions to be addressed:

• Add documentation of the new logging options to csaf_downloader.md, including the defaults.
• Add docs about validation subdirectories and behaviour if either validation or forwarding fails.
• in cmd/csaf_downloader/config.go:
  • typo: log download should probably be loag downloads
    • comment typo: // If no special given use current working directory. special what? ;)
    • in cmd/csaf_downloader/forwarder.go:
  • comment typo: suited ti forward
    • in cmd/csaf_downloader/main.go the following slog.Info("No domains given.") should be at least level Warning, if not Error.

[s-l-teichmann]

Solved by #447