Import Export: Add related MRs to list of not exported items

What does this MR do and why?

This updates our [Import/Export docs]https://docs.gitlab.com/ee/user/project/settings/import_export.html#items-that-are-not-exported) by adding Related Merge requests to the list of not exported items.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

N/A

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.

Relates to #385463