Add developer guide

[pleshakov]

As a potential contributor to the NGINX Kubernetes Gateway
I want a developer guide to explain how to build and test my contributions
So that I do not have to spend an excessive amount of time figuring out how to contribute.

Acceptance

• A developer guide is created within the NKG project that at minimum contains:
  • How to setup the development environment
  • How to build and test the project
  • How to implement a feature
  • A code style guide
  • Information on the architecture and design of the project

### Tasks
- [ ] https://github.com/nginxinc/nginx-kubernetes-gateway/issues/670
- [ ] https://github.com/nginxinc/nginx-kubernetes-gateway/issues/671
- [ ] https://github.com/nginxinc/nginx-kubernetes-gateway/issues/672
- [ ] https://github.com/nginxinc/nginx-kubernetes-gateway/issues/673

[mpstefan]

Need to add style guide to this story