Social Marketing Handbook

Ref:

• https://gitlab.com/gitlab-com/marketing/issues/426
• https://gitlab.com/gitlab-com/marketing/issues/386
• https://gitlab.com/gitlab-com/marketing/issues/524

This MR is a WIP and it depends on https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3061 to get merged before this one does.