Document how to convert configuration from NIC to NGF

[sjberman]

As a user of NGINX Ingress Controller and potential user of NGINX Gateway Fabric
I want a guide that shows me how to convert my configuration expressed in Ingress, VirtualServer, VirtualServerRoute, and annotations to NGINX Gateway Fabric resources with the same configuration
So that I can understand how Gateway API translates the configuration I already have
And so that my path to using Gateway API in my environment is easier.

Acceptance

• Write a new document on how to convert NIC configuration to NGF configuration that includes:
  • The purpose of the conversion is and when it is meant to be used.
    • A disclaimer is placed in a prominent location that explains that this tool is an assistant and the user is responsible for verifying that the configuration is what you expect it to be and what you ultimately apply to your cluster.
• A link is put in our README that directs users to this guide.
• A link in our docs to point to this guide, probably in the Get Started guide.

Dev Notes

• This could involve utilizing https://github.com/kubernetes-sigs/ingress2gateway for some of it