docs: clarify setup guide deploys single-node workspace

[saadqbal]

The setup guide does not explicitly state that the installer deploys a single-node workspace on one machine. Users have to infer this from scattered cues ("any modern machine", "local Kubernetes cluster", etc).

Added a <Note> callout near the top of environment-setup/setup-guide.mdx making the single-node constraint explicit and pointing users to the EKS deployment guide for multi-node setups. Also reinforced "one host per workspace" in the Requirements section.