Troubleshoot repository mirroring pipelines

What does this MR do?

If CI/CD for external repositories stops triggering pipelines, it might be as simple as not having ticked that option when reconfiguring repository mirroring.

As recommended by the customer in a ticket (internal link) adding something to the troubleshooting about this.

Also the other ideas we came up with in the ticket.

I think CI/CD for external repositories uses pull mirroring since its premium, and the mirroring is clearly configured in GitLab, not in (say) GitHub. Given the presence of this configuration, it has to be either pull mirroring or push mirroring, and in this context it's definitely not push mirroring.

The confusion arises from the presence of webhooks for the Github integration, and this is needed for pipelines for pull requests.

This being the case, the documentation for all this functionality is in multiple places:

• https://docs.gitlab.com/ee/user/project/repository/mirror/pull.html#configure-pull-mirroring
• https://docs.gitlab.com/ee/user/project/repository/mirror/
• https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/
• https://docs.gitlab.com/ee/ci/ci_cd_for_external_repos/github_integration.html

I've tried to make this findable when using CI/CD for external repositories, while also documenting it for pull mirroring.

Related issues

#346630

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