Carved out of OSSM-8200.

The concepts around scaling and multi-tenant topologies in OSSM 3 are different than in OSSM 2, and more closely mapped to upstream Istio. This issues is to document concepts around service mesh scaling & multi-tenancy for OSSM 3 as they would relate to a typical Red Hat enterprise customer.

Early work: https://docs.google.com/document/d/15N5vQxp6xr3lH2lRG50feIPo4xANwA-0JiMN6m_Vecc/edit?usp=sharing 

This will be published to the openshift-service-mesh Github repo for review prior to being ready for product documentation.