docs: add cluster scaling guide for VM installs

[jlav]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Customers on the VM install want to scale past a single machine, mainly for sandbox capacity. Nothing in the docs tells them how, so this adds a Scaling the Cluster page under VM Install.

It's written in terms of machines and roles rather than nodes and scheduling, since that's the layer replicated presents to customers. The main thing it recommends is running sandboxes on dedicated machines in production, so sandbox load can't eat capacity the application needs.

Two smaller points I made deliberate calls on:

• There's a warning about network reachability between machines. It's the one semi-technical bit I kept, because a machine that can't reach the others joins fine and then silently fails to run anything.
• The app-machine section spells out that going from one machine to two buys capacity but no extra resilience, and three is the first count that survives a failure. Framed without k8s terms.

Also adds the new Run sandboxes on dedicated nodes setting to the Sandbox Configuration table on the Admin Console page.

That setting ships in OpenHands/OpenHands-Cloud#1133 through OpenHands/OpenHands-Cloud#1136.

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