docs: create "Managing your cluster" guide

[steverfrancis]

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Once you have followed the Sidero guides, and "Created your first cluster", people will want to know how to:

• add/shrink the nodes in a running cluster.
• destroy said cluster, and what happens
• report on running clusters, and available/unallocated nodes
• upgrading things. Certainly upgrading Sidero. Upgrading K8s? (don't know if that is anything to do with Sidero - maybe that's just a pointer to some Talos OS docs.Or K8s docs.)