Add documentation on creating and using service classes

[mcmire]

Currently, teams are using outdated patterns, such as handleFetch from controller-utils, to make HTTP requests within controller packages. The service class pattern is superior to this approach, because it allows engineers to:

• automatically retry failed requests
• pause for a period of time if the API is down before retrying again
• easily implement other patterns such as batching
• prevent clients from needing to adapt to breaking changes in APIs
• use the messenger to make HTTP requests within UI components in the same way that controllers would

We should add proper guidance for teams.

Acceptance Criteria

• A guide on service classes exists in docs/ that covers:
  • Why service classes are preferred over simple functions
  • How to create a service class
    • Why creating an abstract type is important
    • How to use createServicePolicy in service classes that make HTTP requests
  • How to use the service class in a controller
  • How to expose methods of the service through the messenger
• Guidelines exist in the "Writing Controllers" document that advises engineers to use service classes, and links to the Service Classes guide

Prerequisites

• Reduce boilerplate for actions that expose controller methods through the messenger #4582