CONTRIBUTING Documentation Coverage

[shaneutt]

Spike

For the KIC 2.0 release we intend to make major architectural changes to the KIC which will fundamentally change how contributing to KIC works, as such we need to change our CONTRIBUTING.md and other development docs in support of this.

Acceptance Criteria

• document the basic usage of Kubebuilder SDK for creation and maintenance of APIs and link to important topics in upstream docs
• define the KEP process, and how this should be used when trying to establish new APIs and new major features. Maybe it's enough to link to https:/Kong/kubernetes-ingress-controller/blob/next/railgun/keps/README.md. @eskibars suggests that we add a "KEP proposal" issue template.
• provide context regarding contributing new APIs and link to feature gates documentation we're not sure about the course regarding feature gates - see Spike: Feature gates #1237