Docs: document how remote write API works

[pracucci]

We're seeing some people asking how they can build custom clients to write metrics to Mimir. There's foundamentally a lack of information on how the remote write API works in our documentation. We should explain:

• Purpose of the remote write API
• The request body format
• Link a reference implementation (e.g. in golang we could show https://github.com/grafana/mimir/blob/29142fc7ae465e31478483d818048070143ee688/pkg/continuoustest/client.go#L176)
• Write proxies (e.g. https://github.com/grafana/influx2cortex/) as an alternative to easily remote write to Mimir using simpler protocols

[osg-grafana]

Who are the SMEs (subject matter experts) for this topic?

[huy7070]

Has any SME been able to work on this? I have been trying to send data via curl, but not sure if it is even possible with just curl alone.

[LeviHarrison]

No, not practically. The remote-write payload is a protobuf object which needs to be serialized.