Rewrite NginxGateway CRD doc to reflect auto-generated API reference doc

[pleshakov]

#1884 introduced auto generated api reference CRD docs

https://docs.nginx.com/nginx-gateway-fabric/how-to/configuration/control-plane-configuration/ doesn't reference docs generated for ControlPlane CRD.

Acceptance criteria:

• Make sure API reference docs are referenced
• Remove any redundant content

[github-actions]

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