DOCS-3352 GitLab CI/CD Integration Documentation

[alai97]

What does this PR do?

Adds doc page for GitLab under CI/CD Integrations.

Motivation

DOCS-3352

Additional Notes

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/alai97/synthetics-cicd-integrations-doc-updates/synthetics/cicd_integrations/gitlab

Modified Files

• https://docs-staging.datadoghq.com/alai97/synthetics-cicd-integrations-doc-updates/synthetics/cicd_integrations/