Add basic documentation on service classes

[mcmire]

Explanation

As more and more people make changes to this repo, the need to understand services has grown.

This new document offers a tutorial that explains what a service does, how to create a service, and how to use it in a project.

References

Progresses #5741.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes