docs: add Default Ports and Stopping the Node sections to README

[benperlmutter]

Summary

This PR adds two new documentation sections to the README to help new users:

1. Default Ports - Documents all exposed ports for the Execution Client and OP Node
2. Stopping the Node - Explains how to gracefully shut down the node

Changes

• Added "Default Ports" section with tables documenting:
  • Execution Client ports (8545, 8546, 7301, 30303)
  • OP Node ports (7545, 9222, 7300, 6060)
• Added "Stopping the Node" section with:
  • docker compose down for graceful shutdown
  • docker compose down -v for full reset
  • Note about Ctrl+C for foreground shutdown

Issues

Closes #903
Closes #904

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Pull Request opened by Augment Code with guidance from the PR author

[cb-heimdall]

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