Add clarification to `Self-Service Performance Regression Testing` page

- **Master pipeline**: [Latest successful builds](#capturing-an-image-url-from-a-master-pipeline)

- **Release candidates**: Pre-release builds for upcoming versions

- **Docker Hub**: [hub.docker.com/u/gitlab](https://hub.docker.com/u/gitlab)

- **Dev Builds**: [dev.gitlab.org/gitlab/omnibus-gitlab](https://dev.gitlab.org/gitlab/omnibus-gitlab/-/pipelines)

**Note:** ⚠️ If you're getting the Image URL from the web pipeline artifacts page, you need to convert the URL to use the API endpoint

see [Capturing an image URL from a Master pipeline](#capturing-an-image-url-from-a-master-pipeline) or [Capturing an image URL from a branch pipeline](#capturing-an-image-url-from-a-branch-pipeline) to capture the URL and then convert:

```plaintext

https://<URL>/gitlab/omnibus-gitlab/-/jobs/<JOB_ID>/artifacts/file/pkg/ubuntu-noble/...

```

to

```plaintext

https://<URL>/api/v4/projects/gitlab%2Fomnibus-gitlab/jobs/<JOB_ID>/artifacts/pkg/ubuntu-noble/...

```

You can test the URL with

```bash

curl -H "PRIVATE-TOKEN: $PRIVATE_PROD_TOKEN" \

  "https://dev.gitlab.org/api/v4/projects/gitlab%2Fomnibus-gitlab/jobs/<JOB_ID>/artifacts/pkg/ubuntu-noble/<PACKAGE_NAME>.deb" \

  -o /tmp/test.deb -L

file /tmp/test.deb  # Should show "Debian binary package"

```

more information in the [Prepare GET Configuration](#prepare-get-configuration) section

**Document:**