docs: suggest copy linking for GitLab integration guide (#11067)
## Summary
Hardlinking does not work in that context and raises a warning.
Setting the link mode to copy makes the warning go away.
## Test Plan
Tested on gitlab.com and our self-hosted GitLab instance.
Before changing the link mode:
https://gitlab.com/fgreinacher/uv-test/-/jobs/8986967570
After changing the link mode:
https://gitlab.com/fgreinacher/uv-test/-/jobs/8987026307.
:hammer_and_pick: with :heart: by
[Siemens](https://opensource.siemens.com/)
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Co-authored-by: Zanie Blue <contact@zanie.dev>
