Document GitLab support in onboarding setup wizard

[mintlify]

Summary

The dashboard setup wizard now supports GitLab as a Git provider during onboarding, in addition to GitHub. Users can select GitLab, download their content as a zip, and connect a GitLab repository directly from the wizard.

Changes

• Updated the quickstart to mention GitLab as a provider option during onboarding
• Added a tip to the GitLab deploy page noting that the setup wizard handles these steps automatically for users who selected GitLab during onboarding
• Updated the "Clone to your own repository" section on the GitHub page to reference both GitHub and GitLab setup paths

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Note

Low Risk
Documentation-only changes that adjust onboarding/setup instructions; low risk beyond potential user confusion if links or wording are incorrect.

Overview
Updates onboarding documentation to reflect GitLab as a first-class Git provider option alongside GitHub (including revised Quickstart wording and the “skip connecting” onboarding path).

Clarifies post-onboarding migration guidance by splitting GitHub’s “clone to your own repo” step into provider-specific setup directions, and adds a tip on the GitLab deploy page noting the onboarding wizard can walk users through GitLab setup automatically.

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[ethanpalm]

closing for #5417