Add documentation for using pre-existing volumes in helm

Created by: caugustus-sourcegraph

The helm chart documentation lacks any instructions or examples on how to use pre-existing volumes in helm.

For deployments, set the volumeName override value for each specific deployment. The PV resource must already exist.

For statefulsets, create PV's that match the given storageSize requirements and set the claimRef values equal to the pvc names being generated (e.g. repos-gitserver-0, grafana-data-grafana-0, data-indexed-search-0, increase based on replica count for each). https://cloud.google.com/kubernetes-engine/docs/how-to/persistent-volumes/preexisting-pd shows how to create a PV with a claimRef.

Context: https://sourcegraph.slack.com/archives/C02E4HE42BX/p1656343305088499