Add details on injecting variables into a workspace

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125542



Merged-by: Grzegorz Bizon <grzegorz@gitlab.com>
Approved-by: David O'Regan <doregan@gitlab.com>
Reviewed-by: Grzegorz Bizon <grzegorz@gitlab.com>
Reviewed-by: Chad Woolley <cwoolley@gitlab.com>
Reviewed-by: Vishal Tak <vtak@gitlab.com>
Co-authored-by: Vishal Tak <vtak@gitlab.com>