Create docstring descriptions of purpose and interface for SonadorServer, SonadorImagingServer, and SonadorServerManager

There is a degree of ambiguity in what SonadorServer, SonadorImagingServer, and the SonadorServerManager` classes do. It would be nice to have additional notes on how these systems work and the role that they play within the Sonador/Orthanc plugin.

• sonador.servers.SonadorServer: provides a connection to the Sonador web application and provides models for Orthanc resources. The sonador is the same library used for external integrations.
• sonador.servers.SonadorImagingServer: provides a representation of "Orthanc" and servers as the primary entry point for the Orthanc API. Used internally by the plugin for synchronizing the local Orthanc configuration with the Sonador web application.
• SonadorServerManager: class which manages callbacks and timers within the Sonador/Orthanc plugin. Provides methods to register and invoke callbacks.