[Serve] Document max_replicas_per_node deployment option

[jjyao]

Why are these changes needed?

Add documentation for max_replicas_per_node deployment option

Related issue number

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[edoakes]

Please add:

• A description of what the default scheduling policy is and why it might be problematic.
• Make it clear that the behavior is only within a deployment and multiple deployments can still share resources.
• A concrete code example of an application where this is set. Maybe let it have 2 deployments and set max_replicas_per_node=1 on one, max_replicas_per_node=2 on the other. Then describe how these would be scheduled.

[edoakes]

Looks great!