Skip package-and-test pipeline on documentation merge requests

As part of the maintenance policy extension (gitlab-com/gl-infra&828 (closed)), the package-and-test pipeline is automatically executed on merge requests targeting stable branches as a way to ensure the code changes introduced into these branches are Quality compliant. The package-and-test pipeline is a heavy one in terms of resources and time.

CI resources could be saved and engineering productivity could be increased by skipping the execution of this pipeline on merge requests introducing documentation changes only. Care should be taken to only skip it if the MR is documentation only, if the MR introduces code changes and documentation, the package-and-test pipeline should definitely be executed.