doc: Explicitly state Gitaly support status in K8S and similar envs

What does this MR do and why?

Document loud and clear that Gitaly Cluster is not ready for Kubernetes, Amazon ECS or similar environments. Note related known limitations in the, well, known limitations section of the docs.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.