docs: Add link to changelogs info
Description
User feedback:
config-file option is mentioned and the default of changelog config file is described but no link or docs about the content of this config file.
This change adds a reference link to the changelog configuration documentation. When users see the help text for the --config-file option in the changelog generate command, they'll now see a link to GitLab's official documentation about changelogs, making it easier for them to learn how to properly configure this feature.