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

[shannonxtreme]

From #664 - the CI manifest is a good source of info for a "Install the GCP provider" task: https://github.com/kubernetes/cloud-provider-gcp/blob/25e5dcc715781316bc5e39f8b17c0d5b313453f7/deploy/packages/default/manifest.yaml

[shannonxtreme]

Content plan here: https://docs.google.com/document/d/1NuY8PAIw41SHNfuOoiuBqmWGnn-h0fhZt2MpxtWN_nw/edit?usp=sharing

[BenTheElder]

/triage accepted
/priority important-soon
(TBH I could about call this critical-urgent, but I think that's reserved for "clusters are on fire/vulnerable right now", this will be critical-urgent by the end of 1.31 cycle if we're not there yet)

Thank you @shannonxtreme 🙏

[shannonxtreme]

Plan is approved, drafted the content for conceptual section: https://docs.google.com/document/d/1RLb3V-ci-Sz54C4jh8ucPRFnIBpNXewmoQ3PMmV0RNY/edit