docs: add basic node health checks

[juank73]

• Add a short "Verify the node is running" section after the Quick Start commands.
• Gives operators simple docker compose and curl checks to confirm that the Base node stack is up and the L2 RPC is responding.

[cb-heimdall]

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[juank73]

In the future we could also add a short note about checking optimism_syncStatus to see how far behind the node is, similar to the example in the Base node docs, but the current basic health checks are already useful.