Document Export License Usage button

What does this MR do and why?

To make it easier for air-gapped customers to provide user usage information to sales for renewals or true-ups #213615 introduced the "Export license usage file" button.

This is the follow up documentation.

Screenshots or screen recordings

Does not apply.

How to set up and validate locally

Does not apply.

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.

Related to #235885