Add GitLab references to Changes Detection docs

[andrea-mosk]

What does this PR do? What is the motivation?

We add the same GitLab references of this PR to the changes detection docs (they'll be merged together).

Merge instructions

Merge readiness:

• Ready for merge

Merge queue is enabled in this repo. To have it automatically merged after it receives the required reviews, create the PR (from a branch that follows the <yourname>/description naming convention) and then add the following PR comment:

/merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/andrea.moscatelli/add-gitlab-to-changes-detection-docs/continuous_delivery/features/code_changes_detection

[henrywalsh]

LGTM!

Not for this PR, and perhaps it's not worth it, but since there's so much cross over between this page and the DORA setup page, I wonder if we could centralize the shared information somehow. 🤔