feat: Add docs to install Nginx Gateway Fabric with N+

[salonichf5]

Proposed changes

1. The MR contains documentation updates for installing Nginx plus with Nginx gateway fabric using

• JWT token
• Setting up Docker registry.

2. This PR adds additional fields - imagePullSecretName & imagePullSecretsNames in values.yaml to specify secrets associated with the docker registry to pull and push images to.

Information has been updated for installing Nginx Plus with Nginx Gateway Fabric using Helm charts and manifests.

Problem: User should have documentation to install/upgrade Nginx Plus with Nginx Gateway Fabric

Solution: Added documentation specifying multiple ways to install plus with Nginx Gateway Fabric using both helm chart and manifests.

Testing: Lint checks and pipeline builds

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #ISSUE

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works N/A
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[sjberman]

Still a draft, so I'm assuming you're still working on it, but you're missing the actual installation steps. If you look at the example NIC links in the ticket, and scroll down to the installation steps, you'll see there are options for both OSS and Plus. We need a similar structure for all of our installation steps.

[sindhushiv]

@sjberman Do we not need approval from the documentation team for it?

[sjberman]

@sindhushiv yes, and they are tagged as reviewers on the PR. It's still a draft though, so it's not urgent.

[ADubhlaoich]

I'm approving for posterity but have made a humongous amount of edit suggestions.

I can clarify the design changes for each suggestion if desired: I anticipate I will make further changes in the future, but it's beyond the scope of the PR contents.