Add a default disk buffer config for quickstarts

[gaylatea]

We've made a fantastic start with the quickstart but I think that we can add a default disk buffer to close the gap between POVs and production deployments.

I think this involves two steps:

• [ ] Add a persistence block to the default values that sets up a volume with our default recommended buffer size, which is 48 GiB. I think you
• [ ] Add a buffer block to the datadog_logs sink in the default config, which will utilize the created PVC with the default size.

Open Questions

• Should we also create a buffer for datadog_metrics in the default config? I would assume so, but I'm torn on whether we should double the PVC size or carve out from the datadog_logs buffer.

[spencergilbert]

We did some discussing and have some updates to the going to production docs around these values, so I'll wait until that stabilizes before adding this 👍