Merge branch 'dz/451478-add-initial-purchase-docs-for-duo-pro' into 'master'

Add documentation for initial DuoPro purchase

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148180



Merged-by: Kati Paizee <kpaizee@gitlab.com>
Approved-by: Kati Paizee <kpaizee@gitlab.com>
Reviewed-by: Fiona Neill <fneill@gitlab.com>
Co-authored-by: dzubova <dzubova@gitlab.com>
Co-authored-by: Fiona Neill <fneill@gitlab.com>