Add Fabric CI/CD GitLab sample

[elenaterenzi]

Type of PR

• Documentation changes
• Code changes
• CI-CD changes

Purpose

• Add a new Fabric CI/CD sample for GitLab and multi-tenancy

Does this introduce a breaking change? If yes, details on what can break

Author pre-publish checklist

• [ ] Added test to prove my fix is effective or new feature works
• [x] No PII in logs
• [x] Made corresponding changes to the documentation

Validation steps

• ...

Issues Closed or Referenced

• Closes #677

[promisinganuj]

Another note about not renaming the "Deployment Pipelines" in the diagram to "Release Pipelines" or something else to avoid confusion with Fabric deployment pipelines.