Need documentation on deploying and managing airgapped clusters

[Tejeev]

I think some of the main points to hit would be:

• Where to set the registries so that the downstream cluster will pull rancher and other images from a designated private registry.
• What the current limitations are.
• Should fit into the flow of someone following our docs step by step to deploy a cluster. For example (but not saying this is the best way): Have a group under Cluster Deployment with a page on deploying air-gapped clusters for each cluster type, and have links where a customer following a step by step deployment of their cluster will see that if they are airgapped they should follow 'this doc' before proceeding.
• Not everyone who is airgapped use the same term to mean airgapped or sometimes even understand what it means so we should define our useage or say something like "If your cluster is isolated from docker.io" which specifically says what we mean rather than just keep using "air-gapped" in the intro of the main page on the topic.

[Tejeev]

Related correction to RKE2 docs: #86