Add handbook reference for maximum import size

What does this MR do and why?

~"group::import" has received quite a few questions recently related to the maximum import size for projects. This MR adds a reference to the Account and limit settings page for the project import/export handbook page.

Screenshots or screen recordings

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.

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