Document use of Correspondence in an Altinn app

[martinothamar]

Document how to enable and use Correspondence as a product in an Altinn app

Acceptance criteria

• Reference page for reference docs
• User guide including a more complete example (for example sending a correspondence within IProcessTaskEnd)
• Semantics of the messaging is clear - failure modes, developer responsibilities
• Update metric registry/docs for telemetry output (how to monitor how this functions)