Intro tutorials for MCS API use cases

Develop some vendor agnostic intro tutorials about the MCS API that can be easily digested as part of a generic documentation push. Keep them implementation agnostic but I think it would be useful to tie them to some common use cases as suggested below.

Ideas:
* MCS for a single-cluster stateful service serving multiple clusters (single ServiceExport case) (see [this walkthrough](https://cloud.google.com/kubernetes-engine/docs/how-to/migrate-gke-multi-cluster#migrate_to_multi-cluster_services) for some inspo)
* MCS for a same-named stateless service in clusters in different regions for simple failover (merged ServiceExport case)
* MCS for a same-named stateless service for blue-green upgrade (see [this video](https://youtu.be/hztXW7Hk6Vo) for example)

Start with the three MCS API use cases listed in the issue, reviewing the linked GKE walkthrough and blue-green example for context. Draft vendor-agnostic introductory tutorials that explain each use case without implementation-specific steps, and confirm that all three scenarios are covered and readable as generic documentation.

Written by the indexing model from the issue text.