Docs process

Related:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/29654
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#definition-of-done
• https://docs.gitlab.com/ce/development/writing_documentation.html#writing-documentation
• https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/5520