Add documentation on creating and using data services

[mcmire]

Currently, teams are using outdated patterns, such as handleFetch from controller-utils, to make HTTP requests within controller packages. The data service 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 data service exists in docs/ that covers:
  • Why classes are preferred over simple functions
  • How to create a data service class
    • Why creating an abstract type is important
    • How to use createServicePolicy in service classes that make HTTP requests
  • How to use the class in a controller
  • How to expose methods of the data service through the messenger
• Guidelines exist in the "Writing Controllers" document that advises engineers to use service classes, and links to the guide being written here

Prerequisites

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

[mcmire]

I've created a PR that shows engineers how to create and use a service class in a project: #5858

This is probably good enough for now, but it would good to:

• Document more reasons for why a service is useful
• Document what createServicePolicy does
• Add a link from the controller guidelines to the documentation on services