[DRAFT] fix(ai-gateway): update the AI Gateway V2 migration guide to use the new tooling

[gperanich]

Description

Fixes #6026

• Added a prerequisite noting identity providers, vaults, and per-model ACLs must be prepared by hand (converter can't recover them from decK)
• Expanded migration overview from 5 to 6 steps and updated the mermaid diagram to show the new manual step
• Added a note to Step 2 that auth plugins (key-auth, openid-connect) on AI Model routes are stripped during conversion
• Inserted a new Step 4: Add identity providers, vaults, and ACLs with YAML examples for identity_providers, vaults (plus a config-store caveat), and access.acls
• Renumbered old Step 4 → Step 5 (kongctl control plane config) and old Step 5 → Step 6 (apply)
• Added an "Identity" bullet to the AI Models verification checklist
• Added an "Authentication" bullet to the AI MCP Servers verification checklist clarifying their auth plugins do convert automatically (unlike AI Models)

Preview Links

https://deploy-preview-6457--kongdeveloper.netlify.app/ai-gateway/v2-migration-guide/

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

[netlify]

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