Improve dual-stack documentation

[1player]

Is your feature request related to a problem? Please describe.

• The template shown at https://docs.k0sproject.io/v1.24.3+k0s.0/dual-stack/ should specify spec.network.provider = calico for it to work. Otherwise the cluster will be using kube-proxy.
• The template mentioned above says "Use the following k0s.yaml as a template", but it's only listing the spec object. Since k0s seems to accept partial configuration (see #2090), replace the example with an actually valid k0s.yaml file (with apiVersion, kind fields, etc.)
• In the configuration docs, there is no mention of spec.network.calico.mode accepting the option bird. Only vxlan and ipip are shown to be acceptable.

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