Update docs for Grafana Cloud support

[cyberbit]

A quick review of Grafana Cloud options has revealed that the options in documentation for HTTP pushing have changed, making the guide out of date. Need to review how to best support going forward.

New docs appear to be here https://grafana.com/docs/grafana-cloud/send-data/metrics/metrics-influxdb/push-from-telegraf/

[cyberbit]

Existing adapter seems to still be working using the linked docs. Token setup happens in the account dashboard now instead of the instance dashboard.