Document use of migrator helm chart when migrating to external databases

Created by: caugustus-sourcegraph

The migrator helm chart can be used to initialize an external database. The README for that chart documents how to use it, but this isn't mentioned in any of the helm docs.

https://docs.sourcegraph.com/admin/deploy/kubernetes/helm#using-external-postgresql-databases and https://docs.sourcegraph.com/admin/external_services/postgres#postgres-permissions-and-database-migrations should be updated to include a reference to this chart and instructions on how to use it, similar to the kustomize instructions.

Full context: https://sourcegraph.slack.com/archives/C02E4HE42BX/p1653566946137919