GitLab Integration documentation (#6857)
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## Summary
Documentation for GitLab integration, reliant on the new tags introduced
in https://github.com/astral-sh/uv/pull/6053
## Test Plan
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Co-authored-by: David Fritzsche <9479371+davidfritzsche@users.noreply.github.com>
Co-authored-by: Zanie Blue <contact@zanie.dev>
