Merge branch 'dz/7634-add-docs-for-duo-pro' into 'master'

Add documentation how to purchase Duo Pro

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142314



Merged-by: Michael Lunøe <michael.lunoe@gmail.com>
Approved-by: Laurena Alves <lquinteiroalves@gitlab.com>
Approved-by: Michael Lunøe <michael.lunoe@gmail.com>
Reviewed-by: Michael Lunøe <michael.lunoe@gmail.com>
Co-authored-by: Tatyana Golubeva <8110789-tgolubeva@users.noreply.gitlab.com>
Co-authored-by: dzubova <dzubova@gitlab.com>