Flux contributor documentation: Controller fundamentals

[scottrigby]

Problem:
We have consistently heard that the bar is very high to contribute to Flux controllers, and the requirements are not clearly defined in any single place.

Proposed solution:
We can help people who want to contribute to Flux Controllers by providing guidance in a new docs section on Flux Controller contributor documentation.

Goals:

• This would be new documentation on fluxcd.io
• Encourage new contributors to Flux Controllers
• Encourage new controllers built on top of Flux packages (GitOps Toolkit)

Note: we may want to draw inspiration from this page: https://cluster-api.sigs.k8s.io/developer/testing.html

[dholbach]

https://fluxcd.io/contributing/flux/ now has links to dev docs of the individual controllers. Does that help or do we need to do more?