[Rancher2] Document that users should not use v1 API

[sirredbeard]

We need to document that users should not use API v1. They should automate using kubectl directly.

@cbron will provide some technical background on why and how.

[cbron]

While the v3 api was the primary entry point into the rancher application, v1 is instead a simple layer on top of the k8s api. Anything a user wishes to do with v1, they should do with the k8s api instead. Users should also not be using the v1 API as we do not provide any backwards compatibility guarantees. It should instead be seen as an implementation detail that Rancher can break/alter at any time.

I can provide more details as needed.

[catherineluse]

Related issue https://github.com/rancher/rancher-docs/issues/61

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[btat]

Consolidating into https://github.com/rancher/rancher-docs/issues/583.