feat(ai-gateway): Missing Vertex AI provider documentation

[TuannasCodeCatastrophe]

Description

Missing Vertex AI provider documentation
We need to fill in the gaps in the AI Providers landing page. We're missing how-to guides for setting up AI Proxy and AI Proxy Advanced for Vertex AI chat routes.

DoD:

How-to for using Vertex AI with AI Proxy similar to this guide
How-to for using Vertex AI with AI Proxy Advanced similar to this guide
Vertex AI section on the AI Providers page updated with a how-to block:
Make sure all provider tags are in the allowed tags list. If not, add them.

• header:
  type: h3
  text: "Vertex AI"
  columns:
  • blocks:
    • type: card
      config:
      title: AI Proxy with Vertex AI
      description:
      Configure the AI Proxy plugin to proxy requests to a Vertex AI model
      cta:
      url: /plugins/ai-proxy/examples/mistral-chat-route/
      align: end
      text: Configuration example
  • blocks:
    • type: card
      config:
      title: AI Proxy Advanced with Mistral
      description:
      Configure the AI Proxy Advanced plugin to proxy requests to a Mistral model
      cta:
      url: /plugins/ai-proxy-advanced/examples/mistral-chat-route/
      align: end
      text: Configuration example
• header:
  type: h4
  text: How-to Guides
  columns:
  • blocks:
    • type: how_to_list
      config:
      plugins:
      - ai-proxy
      - ai-proxy-advanced
      tags:
      - vertex-ai
      quantity: 3
      allow_empty: true

Fixes #3634

Preview Links

https://deploy-preview-3935--kongdeveloper.netlify.app/how-to/set-up-ai-proxy-advanced-with-vertex-ai/
https://deploy-preview-3935--kongdeveloper.netlify.app/how-to/set-up-ai-proxy-with-vertex-ai/

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).

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[tomek-labuk]

Loons good, make sure to apply my suggestions