Update artifacts keyword to new YAML reference style

As part of &6941 (closed), we need to update the artifacts keyword (and subkeys) to align with the style defined at https://docs.gitlab.com/ee/development/cicd/cicd_reference_documentation_guide.html.

For more examples, see the already completed MRs listed in the epic.

The goal is to make this page a purely reference page, with all concept and task details on more appropriate pages related/dedicated to each feature.