Add v2 to v3 migration guide

[nginx-seanmoloney]

Proposed changes

• Add v2 to v3 migration guide

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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[fabriziofiorucci]

Migrate NGINX Agent running in containers - I presume this refers to the case where I have a pod running NGINX and the Agent, and the Agent's config file is mounted through a ConfigMap. Am I correct here? In case I am, it might be worth mentioning this in the MD file