Docs: Improve N+ Images Docs

[kate-osborn]

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Instructions on how to get N+ images are split across two docs:

• https://docs.nginx.com/nginx-gateway-fabric/installation/ngf-images/jwt-token-docker-secret/
• https://docs.nginx.com/nginx-gateway-fabric/installation/ngf-images/pulling-ngf-image/

which makes it hard to follow.

Additionally, the "Pushing an NGINX Plus image to a private registry" doc also contains instructions on how to pull the image.

A/C:

• Consolidate N+ images docs into a single doc