Further clarification of types of account access, and how to gain access

Further clarification per the discussion here: https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/2284#note_12666107

\cc @brittanyr @rspeicher if this is merged, then there should be a secure note per service, e.g. one called Slack, one called dev.gitlab.org account, etc. listing who to contact for access. For these particular examples, since they are so common, it is probably OK to stay a bit DRY and refer back to the general-onboarding checklist.

The idea behind keeping track in the Shared vault for all services, is so that people who are already onboarded but change role to where they need a new access they did not have before, they know where to go for access.