chore(data): add .gitkeep for grafana

When starting the grafana docker compose, grafana throws the following error:

```bash
mkdir: cannott create directory /var/lib/grafana/plugins: Permission denied
GF_PATHS_DATA=/var/lib/grafana is not writable.
```

This change persists the grafana directory in the repo and updates the permissions to allow docker container to write files to it.

Author: dtfiedler