docs: Clarify NAT gateway behavior and precedence of single_nat_gateway vs one_nat_gateway_per_az

[Trish-Muyambiri]

Summary

This PR improves the documentation around NAT Gateway configurations by clearly outlining the behavior of:

• single_nat_gateway
• one_nat_gateway_per_az
• Their interaction and precedence when both are enabled

What Changed

• Updated the NAT Gateway config section in README.md to:
  • Use consistent bullet formatting
  • Include clear, labeled scenarios
  • Add a visible note explaining precedence when both flags are set
• Removed a redundant sentence that was covered by the new warning block

Why

This helps users understand how NAT Gateway settings affect behavior in common setups, especially when optimizing for cost or availability.

Closes #1196

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