Document support for absolute submodule URLs in GitLab Runner

What does this MR do and why?

This provides documentation for Add support for absolute submodule URLs (gitlab-runner!3198 - merged), merged for %15.11.

🛠 with ❤ at Siemens

Screenshots or screen recordings

See Add support for absolute submodule URLs (gitlab-runner!3198 - merged) for implementation

How to set up and validate locally

See Add support for absolute submodule URLs (gitlab-runner!3198 - merged) for implementation

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.