Add an architecture document for the RabbitMQ redesign

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

If not required, please explain in brief why not.

Description

This MR adds a section to the documentation describing the design of the RabbitMQ-based scheduler rework.

Issues addressed

Resolves #375 (closed)