Draft: Document GIT_CHECKOUT_STRATEGY

What does this MR do and why?

Documents the new gitlab-runner variable GIT_CHECKOUT_STRATEGY.

Implementation MR can be found here

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.