If someone is using Dendrite and enabling Grafana, we'll no longer
download Synapse dashboards.

If someone is not using node-exporter, we'll no longer download node
exporter dashboards.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2133

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Also includes the dashboards for Synapse and for Node Exporter.

Again has only been tested on debian amd64 so far, but the grafana docker image is available for arm64 and arm32. Nice.