Add a link and explainer to our MetricsPort Grafana template

It's been a longstanding request from relay operators to get something like a template of the network-health MetricsPort dashboard we use. That one is available at a TPA repo, so it is worth thinking about enhancing our MetricsPort support page accordingly. We could think about moving/mirroring that particular template somewhere else as well while we are at it, taking @gus' idea into account.