Helm: Mention runtime config in "Manage the configuration of Grafana Mimir with Helm"

That article doesn't mention how to manage the runtime config with helm. It's fairly simple, but it's not mentioned anywhere.

```yaml
runtime_config:
  overrides:
    tenant_a:
      ingestion_rate: 123
```

The runbooks also don't mention how to set the tenant overrides config or instante limits in helm. As part of this issue, update the runbooks to link to this article.