Define reliability requirements and document key concepts E3.

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Description:
Define the key reliability requirements for the DIDComm mediator server. This involves identifying essential aspects of reliability, such as uptime targets, fault tolerance levels, and recovery time objectives (RTOs). Document these requirements and outline the key reliability concepts, including definitions for fault tolerance, message delivery guarantees, failover mechanisms, and health monitoring strategies.

Acceptance Criteria:

Documented reliability requirements for the DIDComm mediator server.
Clear definitions and explanations of core reliability concepts.