Merge branch '487377-document-offline-license-setup' into 'master'

Document offline licensed self-hosted setup

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167305



Merged-by: Suzanne Selhorn <sselhorn@gitlab.com>
Reviewed-by: Eduardo Bonet <ebonet@gitlab.com>
Reviewed-by: Patrick Cyiza <jpcyiza@gitlab.com>
Co-authored-by: Patrick Cyiza <jpcyiza@gitlab.com>