[Helm] Test/Document migration path from `grafana/loki` and `grafana/loki-simple-scalable` to the new helm chart

[trevorwhitney]

https://github.com/grafana/loki/issues/6873 will bring the loki helm chart back into the Loki repo, and this new chart will replace the existing grafana/loki and grafana/loki-simple-scalable charts. Therefore, we should make sure the migration from those charts to this one works, documenting any gotchyas.

• Make sure there is a migration path from the existing grafana/loki chart to this new chart
• Make sure there is a migration path from the existing grafana/loki-simple-scalable chart to this new chart