DRY development: A cheatsheet on reusability throughout GitLab

What this post added

This post introduces and details several mechanisms within GitLab for achieving the DRY (Don't Repeat Yourself) principle in development, specifically focusing on CI/CD workflows. It explains and provides examples for `include`, YAML anchors, `extends`, `!reference`, downstream pipelines, and CI/CD variables. Additionally, it covers description templates, project templates, and file templates for standardizing code reviews and project initialization. It also recommends the creation of a "Pipeline Center of Excellence" project for managing reusable CI/CD templates.