adds leadership sync workflow

Why is this change being made?

This MR adds a workflow and section in the support handbook that describes how leadership meetings should flow and be memorialized.

Closes gitlab-com/support/support-team-meta#1991 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what