Fix documentation

[hebelsan]

How to categorize this issue?

/area documentation
/kind enhancement
/platform azure

What would you like to be added:
The documentation in usage.md states that the networks.natGateway section is only available for zonal clusters .zoned=true.
See:

gardener-extension-provider-azure/docs/usage/usage.md

Line 167 in 543b98f

- NatGateway is only available for zonal clusters `.zoned=true`.

However I was able to use it with the following infraConf:

  infrastructureConfig:
    apiVersion: azure.provider.extensions.gardener.cloud/v1alpha1
    kind: InfrastructureConfig
    networks:
      vnet:
        name: Test-vnet-name
        resourceGroup: test-rg-name
      workers: 10.0.0.128/25
      natGateway:
        enabled: true
        idleConnectionTimeoutMinutes: 4
        zone: 2
    zoned: false