Clarify docs on k0sConfig field

[nealf]

The configuration page for standalone mode isn't clear about what needs to be included in the k0sConfig field. It took a little while to realize we needed to add a full ClusterConfig definition like the following instead of just the spec portion:

spec:
  k0sConfig:
    apiVersion: k0s.k0sproject.io/v1beta1
    kind: ClusterConfig
    metadata:
      name: k0s
    spec:
      network:
        provider: custom

It might help others to mention that, and I'm happy to open a PR for that if you'd like.

[jnummelin]

PR very welcome 👍