Downstream projects need to be added to upstream allowlist

What does this MR do?

The example on this page https://docs.gitlab.com/ee/ci/pipelines/downstream_pipelines.html#fetch-artifacts-from-an-upstream-pipeline doesn't work currently and results in a 403 Forbidden error when you try to download the upstream artifact from the downstream pipeline.

See https://gitlab.com/anton/433301-downstream/-/jobs/4740104369 for an example pipeline of this.

To resolve this, it's necessary to add the downstream pipeline to the job token allowlist of the upstream project. Then, the example works as expected.

This MR makes 2 changes:

1. Adds a step to add the downstream project to the upstream's job token allowlist
2. Adds a 403 Forbidden troubleshooting section

More context can be found in https://gitlab.slack.com/archives/C03C9EHQ97V/p1690325107868439

Test

This was tested with:

• Upstream config:

  build_artifacts:
    stage: build
    script:
      - echo $CI_COMMIT_REF_NAME
      - echo "This is a test artifact!" >> artifact.txt
    artifacts:
      paths:
        - artifact.txt
  
  deploy:
    stage: deploy
    trigger: anton/433301-downstream

• Downstream config:

  test:
    stage: test
    script:
      - cat artifact.txt
    needs:
      - project: anton/433301-upstream
        job: build_artifacts
        ref: main
        artifacts: true

Related issues

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