Add guidelines for writing controllers, and add example controller

[mcmire]

We have a lot of misalignment among controllers in this repo: older patterns are being used along newer patterns, and controllers that attempt to use newer patterns do so in an inconsistent way. However, too frequently, contributors create new controllers that continue to use outdated patterns. This leads to much churn and makes it more difficult to bring this repo into alignment.

When it comes to controllers, we do not have any documentation around the standards we would like all contributors to follow. Crucially, we are also missing good examples for people to copy.

Acceptance Criteria

• Documentation is present that outlines guidelines for writing controllers
• A complete example is present that fully demonstrates all of the guidelines working together (we can base this off an existing controller, but it should be a brand new controller)