[Documentation] Explain node pre-reqs

[davidnuzik]

We need to ensure that our documentation clearly states any/all pre-requisites for a node (VM).

For example:

• Hostnames cannot be duplicated, some providers (e.g. Linode) may create machines with just "localhost" as the hostname. If this is the case you must pass the --node-name or $K3S_NODE_NAME env var with the curl script and specify a unique node name.
• Just like with k8s (https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/install-kubeadm/) SWAP should not be utilized (e.g. you may encounter Resource limits does not work #392) (also reference Kubelet/Kubernetes should work with Swap Enabled kubernetes/kubernetes#53533)
  -- Side note: We created curl install script should run k3s check-config and warn users of issues #866 so that the k3s install curl script could warn the user if SWAP is enabled.

Provide any FYIs / Gotchas

• if create master node with --disable-agent flag no node is shown when get nodes because kubelet does not run, this is expected.
• if create worker node (in what scenario?) it will not show as master node when get nodes.

[davidnuzik]

Any pre-reqs such as iptables, and other requirements mentioned in #982 should also be taken into consideration.

[davidnuzik]

• --disable-agent is no longer advertised (flag is still there and usable but hidden in docs and cli. It is not recommended. As such i won't include this.
• SWAP is generally a known thing. K8s states this in official documentation around the kubelet. It could be a candidate for FAQ at some point if enough people complain about it but it doens't belong on the node pre-reqs page in the docs.
• iptables issue is something for the new "Known Issues" section and will be handled separately here: [Documentation] FAQ and Known Issues sections #984