Add specific instruction for upgrading NGF from OSS to Plus

[ciarams87]

Proposed changes

Problem: There is a doc that covers pulling the N+ image, however there is no clear explanation for supported customers at how they would go through the process of moving from the free implementation to the paid implementation (replacing NGINX OSS with NGINX Plus).

Solution: Add sections to the installation docs that covers this use-case

Testing: Ran through the steps locally

Please focus on (optional): This could be a separate doc, but I'm not sure of its value. Upgrading from OSS to Plus is very straight-forward

Closes #1933

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
• 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

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE

[ADubhlaoich]

LGTM besides one small suggestion.