Add a section in the documentation talking about the RabbitMQ BuildGrid work
Context
We're planning to redesign the BuildGrid scheduler to use RabbitMQ for communication and job queuing. Currently that design is documented in various email discussions and slide decks, rather than a central location. We should write something up properly in the docs so that we have a canonical source for the new design.
Task Description
Add a doc about the RabbitMQ work which describes the architecture of rmq-server and has a section for each specific service.
Acceptance Criteria
This doc should give a high level overview of the new architecture with some diagrams along with an explanation of why we're making the switch.