Create documentation for setting up and using the out-of-tree provider

[shannonxtreme]

Related issues: #664 #359

With the in-tree provider becoming obsolete, we need basic documentation that covers the following:

• Key concepts about the out-of-tree provider (what is it, why/when would you use it, components and architecture)
• Basic set up and configuration of the out-of-tree provider (what core components use the --cloud-provider config, IAM requirements, GCP VPC requirements)
• Migration from the in-tree provider (high-level migration overview with best practices to minimize unexpected disruptions)

Collaborators: @aojea @BenTheElder

/assign