sql/kv: enhance multi-region migration docs with replica placement

Andy Woods (awoods187) commented:

In https://github.com/cockroachdb/docs/pull/11057, we plan to write a migration from the old primitive patterns to the new multi-region abstractions (this will also allow us to link to the old patterns for discoverability for existing users of the old patterns without making them front and center for new users). We are creating this guide to assure customers that they don't need to migrate immediately and to help them understand how the new patterns translate and build upon the existing primitives. In the new primitives, we aspire for our users to not need to understand leaseholder and replica placement. However, until we fully realize this goal (which also includes addressing data domiciling) we still need to communicate, especially for users of the primitives, how replicas are placed using the new patterns. We can augment this existing doc with this information to help our current users without overcrowding the main docs pages for new users. As an initial proposal, it was suggested that we add a table for voting/non-voting replica for each table type as well as a reference to check replica placement by using the [replication reports](https://www.cockroachlabs.com/docs/v21.1/query-replication-reports.html).

Jira Issue: DOC-1420