Update GitLab setup docs for wizard flow

[mintlify]

Summary

The dashboard now includes a guided setup wizard for connecting GitLab repositories, matching the existing GitHub wizard experience. Users selecting GitLab are walked through provider selection, content download, and repository configuration in a step-by-step flow.

• Updated the "Set up the connection" section in deploy/gitlab.mdx to describe the new wizard steps: provider selection, content download, and repository connection
• Added a subsection for updating an existing connection after initial setup
• Preserved the existing webhook setup and access token instructions

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Note

Low Risk
Documentation-only changes that adjust setup instructions; low risk beyond potential user confusion if the UI text diverges from the docs.

Overview
Updates the GitLab deployment docs to match the new guided setup wizard in the Mintlify dashboard, adding explicit steps for provider selection, optional content download, and connecting the repo via the wizard fields (instance URL, project ID, token, branch).

Replaces the old manual “set up the connection” instructions with the wizard-based flow and adds a short section on how to update an existing GitLab connection; webhook creation instructions remain unchanged.

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[ethanpalm]

closing for #5417