document remote_write configuration

Document how to configure the remote_write feature in CMO. We will likely expose the prometheus-operator config structure plus some optional simplification.

• New section inside the configuration chapter that describes how to setup remote-write with an example on how it looks like for standard remote write implementation. For both CMO and UWM.
• A small note about implications on setting up remote-write to the overall Prometheus cluster.
• How to configure security/auth (e.g. (m)TLS).
• The API.
• Tuning.
• Proxy configuration (if not supported, then we need a statement).

DoD:

• Write down initial document that the documentation team can use as a starting point for the OpenShift docs.