Docs feedback: Triggering pipelines through the API

https://docs.gitlab.com/ee/ci/triggers/

gives curl examples to https://gitlab.example.com . It would be better to derive the api url from the predefined environment variables (CI_API_V4_URL ? ) to make the examples a) easier to adopt and b) to give more real-world examples (cf. #36833)

Implementation plan

Update the following section to use the CI_API_V4_URL CI/CD variable instead of the hardcoded example server:

• https://docs.gitlab.com/ee/ci/triggers/#use-a-cicd-job