Update e2e docs as downstream pipelines run on a mirror
What does this MR do and why?
This MR update e2e tests document to reflect that pipelines triggered from gitlab-org/gitlab are executed on devkit's mirror project rather than on devkit project itself. We need to run pipelines on a mirror due to limitations explained in gitlab-org/gitlab#480462 (comment 2073065414).
Screenshots or screen recordings
N/A. Docs update
How to set up and validate locally
N/A. Docs update
MR acceptance checklist
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