fix: nginx plus upgrade

[y82]

This PR is a major rework of NGINX Plus upgrade:

1. Intro added: EoSD policy (we support 2 latest releases, anyone should use the latest one)
2. Prerequisites added: make sure your repo, certs, jwt are in place, subscription is active
3. Procedure reworked: now when we're R36, there's no need to point out R32>r33 upgrade, it's moved to a separate section
4. Upgrade Notes section that includes procedures for upgrade breaking updates (R27, R33)
5. Code examples tested, optimized

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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[mjang]

Approving with one question