Add docs for configuring persistence

<!-- TOC depthFrom:2 -->

- [Configuring Solr replicas](#configuring-solr-replicas)

- [Enabling persistence for Solr instances](#enabling-persistence-for-solr-instances)

- [Using external Solr instances](#using-external-solr-instances)

<!-- /TOC -->

behaviour - please see the [values.yaml](../../deploy/kubernetes/chart/values.yaml) for a

full list of available variables.

## Enabling persistence for Solr instances

By default, the ESGF Helm chart configures Solr instances to use local ephemeral storage for the

`SOLR_HOME` directories. This means that if a pod gets rescheduled for any reason, all the data

stored in that Solr instance is lost, which is (probably) OK for testing but clearly not ideal

for a production setup.

In order to provide persistent storage for Solr instances, the ESGF Helm chart leverages

Kubernetes [Persistent Volumes](https://kubernetes.io/docs/concepts/storage/persistent-volumes/)

and [Storage Classes](https://kubernetes.io/docs/concepts/storage/storage-classes/). In this way, the

ESGF Helm chart can specify the required storage using

[PersistentVolumeClaim](https://kubernetes.io/docs/concepts/storage/persistent-volumes/#persistentvolumeclaims)

resources while leaving it up to the cluster operator to attach suitable storage and make it

available using a `StorageClass` (the configuration of storage classes is site-specific and beyond the scope

of this documentation).

To configure persistence for Solr instances, just set the following options:

```yaml

solr:

  persistence:

    # Enable persistence for Solr instances

    enabled: true

    # The storage class to use for Solr volumes

    # If not given, the default storage class is used

    storageClassName: fast-ssd

    # The size of the volume to provision for each type of Solr instance

    # Defaults to 10Gi for each

    size:

      master: 40Gi

      slave: 40Gi

      # This is the default size for replica volumes

      # It can be overridden per-replica if required for large replicas

      replica: 20Gi

```

There are additional options for advanced storage configurations - please consult the

[values.yaml](../../deploy/kubernetes/chart/values.yaml) for a full list.

## Using external Solr instances

If you have existing Solr instances that you do not wish to migrate, or need to run Solr