document existing grafana dashboards to gitbook

[nonsense]

@coryschwartz has done a great job upgrading our infra with prometheus monitoring and default grafana dashboards - k8s, redis, weave, etc.

We should improve the documentation on what available dashboards we have and make sure developers are aware of them - currently we have only https://github.com/ipfs/testground/tree/master/infra/k8s#testground-observability

• [x] add a page on gitbook showcasing all existing dashboards (with screenshots) and how to access them
• [ ] raise up issues with dashboards that are not working so that we either fix them or remove them
• [ ] raise up issues with dashboards that we lack and you think we should have

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ISSUES:

• [ ] etcd dashboard is empty

[nonsense]

I've documented most of the existing dashboards at https://docs.testground.ai/cluster-k8s-runner/monitoring

They will likely change once we address https://github.com/testground/testground/issues/851 , so it makes sense to put this issue on hold until OOM issues are solved.