Update nat_gateway.html.markdown to add ip association examples

[ergihu]

The example Terraform code in azurerm_nat_gateway is missing the ways to associate a public IP address or public IP prefix to the NAT gateway.

[manicminer]

Hi @ergihu, thanks for suggesting this. Whilst the example for azurerm_nat_gateway doesn't represent a complete solution, this would be better added to the documentation page for azurerm_nat_gateway_public_ip_association - and indeed that page already has a complete example.

However, you have raised attention to the fact that this example is a bit redundant, and I think it would be a good idea to clean this up. We can also add a link from the azurerm_nat_gateway page to the azurerm_nat_gateway_public_ip_association page, to better assist users seeking a complete example. Doing it this way will make it easier for maintainers as we typically check upstream when looking to update docs, so this is less likely to be overlooked in future.

[github-actions]

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