docs(contribution-guidelines): add API design best practices

[HasithDeAlwis]

Problem Statement

API development can get messy and unorganized as production services increase in size. Minimizing confusion and enforcing design principals (ie. dependency injection, testing, etc.) will go a long way to maintaining a monorepo API. The goal of documenting best practices for API design is to provide distinct steps any contributor can take to write a new endpoint that we can ensure is bulletproof and bug free (as much as we can)

Goals

1. Enforce naming conventions
2. Testing conventions
3. Implement DRY conventions
4. File structure documented

Notes

Best practices are determined overtime. As we start learning best practices the doc site will be updated. For now, contributors will write down best practice findings in the comments of this PR. Once we've had a group discussion among the core dev team, we'll start the documentation process on our docs site.

[JeremyFriesenGitHub]

any updates on this issue @HasithDeAlwis? are we good to close it?