Multi-project pipelines for streamlined repository workflow

This post introduces and details the functionality of multi-project pipelines in GitLab CI/CD. It explains how to trigger external pipelines from a job using `$CI_JOB_TOKEN` and the Pipeline Trigger API, how related pipelines are visualized in the pipeline graph, and how to download artifacts from other projects. It also provides use cases for automatically testing changes across connected components and triggering downstream packaging pipelines.