docs: add multicluster configuration note

[patrickblackjr]

This PR adds a section about multiple clusters using the Kubernetes provider.

This is more general Terraform knowledge, rather than module-specific, but this was still the first time I'd encountered this so it required some extra research. Thus, I thought it would be handy for other people to have available.

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[patrickblackjr]

This is still awaiting review.