NGF: update Get started guide

[sjberman]

The getting started guide needs to change with the new NGF architecture, specifically regarding the NodePorts. The user no longer needs to create the Service, and instead specifies configuration at installation time.

If this PR addresses an issue on GitHub, ensure that you link to it here:

Related: nginx/nginx-gateway-fabric#3240

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[ADubhlaoich]

LGTM!