Merge branch 'fneill-add-subscription-add-on-page' into 'master'

Add subscription add-on page

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143053



Merged-by: Suzanne Selhorn <sselhorn@gitlab.com>
Approved-by: priyanka palanikumar <ppalanikumar@gitlab.com>
Approved-by: Vamsi Vempati <vvempati@gitlab.com>
Reviewed-by: Fiona Neill <fneill@gitlab.com>
Reviewed-by: Vamsi Vempati <vvempati@gitlab.com>
Reviewed-by: priyanka palanikumar <ppalanikumar@gitlab.com>
Co-authored-by: fneill <fneill@gitlab.com>