Clarify who helps in specific support cases

It's not always easy to know where to get help from Support. We have a page, but it's quite long, so we believe it would have to have a "quick reference".

Closes gitlab-com/support/support-team-meta#4930 (closed)

Notes/to do:

• call out "managing who can open a ticket": https://about.gitlab.com/support/managing-support-contacts/#managing-contacts
• https://gitlab.com/gitlab-com/sales-team/field-operations/enablement/-/issues/1969#note_1302781449