Social mkt 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

Fixes conflicts between https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3455 and https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3441