Document compact empty state region component variant

Problem

Following the discussion in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14349#note_184643601 and the need in https://gitlab.com/gitlab-org/gitlab-ce/issues/57086#note_181276839, a new component variant is about to be introduced in !592 (merged) for compact empty state.

Proposal

Let's add some design documentation about this new component variant in Pajamas.