Clean up integrate with GitLab

What does this MR do and why?

• Makes https://docs.gitlab.com/ee/integration/ look more like a top-level page
• Cleans everything up and adds links to other pages instead of duplicating information

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.