Update index.html.md to link to reseller handbook
Added a link to the reseller handbook in the demand gen handbook per: https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3442#note_16862496
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Added a link to the reseller handbook in the demand gen handbook per: https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3442#note_16862496