Add guidelines for writing controllers

[mcmire]

Explanation

This PR adds guidelines for writing core controllers. I'm releasing it as a draft since I wrote them in a hurry and need to do another pass on them myself, but I'm also seeking early feedback, so feel free to leave some.

References

Closes #4504.

Changelog

(None)

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 highlighted breaking changes using the "BREAKING" category above as appropriate