Add documentation for Kubernetes

[stockholmux]

With the helm chart (valkey-io/valkey#2371) and and the operator (valkey-io/valkey-rfc#28) both being actively worked on, we need documentation that fully explains how to run Valkey on Kubernetes and all the considerations from both a database and operational standpoint.

[ksemele]

Just a reference to the Helm chart docs isn't enough?
I mean, it would be nice to extend the README of the Helm repo and just link it in the Valkey docs...

[stockholmux]

@ksemele Sure, we should start with that but as @zuiderkwast mentioned, the matrix of considerations on the Valkey side is not inconsiderable today and will grow with the k8s operator.

In general, we should make sure there is, a minimum, a doc topic when the helm chart is released. It should be generic enough to be later expanded to cover the operator.

[zuiderkwast]

It's great to collect this on one documentation page.

First need something more than ideas though. Once we have a helm chart and/or operator, we should create this page.