Add static IP requirement to documentation

[saviorallen]

Coming here from k3s-io/k3s#11778

The documentation presently does not mention that a static IPs are a requirement for all k3s nodes. Until and unless this becomes untrue, this requirement should be prominently stated somewhere in the documentation, perhaps on the requirements page. At present all it says is "Two nodes cannot have the same hostname" and while of course this is true, they also cannot change their IPs (or presumably their host names) after joining a cluster.

The need to provide static IPs for each node should be made clear on this page.

If a node is taken offline, when it comes back up it will fail to rejoin the cluster if its IP has changed.

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