Docs: Improve N+ Images Docs

[kate-osborn]

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

[mpstefan]

@ADubhlaoich Is this something the docs team could look at picking up? The content is all technically there, we just need to reorganize for the use case.

[ADubhlaoich]

@mpstefan Yep, adding it to my list.

[ADubhlaoich]

I will incorporate this work into the PR I am doing for #2350, since all of it relates to set-up.