Better Developer Guide for outside collaboration and maintenance

**What would you like to be added**:

This developer guide/doc can include things like:
- Links to the designs for different features, system design diagram
- How eks-a interacts with Cluster API
- How to run and develop eks-a cluster controller locally
- Steps to add new provider
- How to run e2e tests locally
- Coding style
- PR submit format (like including a small design to the design folder when introducing large feature)
- Issue link
- Design docs link
- Description
- Testing Performed
- Code freeze schedule and patterns
- ...

**Why is this needed**:

So that developers in the open source community have a reference point of how EKS Anywhere develops and maintains its codebase.

Start with the design folder and the current instructions for running the eks-a cluster controller and e2e tests locally. Done means a developer guide covers the requested architecture, local development, provider contribution, testing, coding, pull request, and maintenance guidance.

Written by the indexing model from the issue text.