Update grafana dashboard documentation

The source of truth for etcd grafana dashboard is no longer maintained in https://etcd.io/docs/v3.5/op-guide/grafana.json. The new source for this is https://github.com/etcd-io/etcd/blob/main/contrib/mixin/mixin.libsonnet.

We should remove the no longer maintained json file and update the grafana instructions to explain how to use the new Jsonnet file.

We need to decide if we just link to the markdown page here https://github.com/etcd-io/etcd/tree/main/contrib/mixin on the website, or shift that content into the website, we should just maintain one source of truth for setting up etcd grafana dashboards.