feat(ai-gateway): Route OpenAI SDK requests to multiple providers

[tomek-labuk]

Description

Fixes #issue This PR also removes an incorrect configuration sample - this configuration requires two instances of an AI Proxy Advanced plugin at two different routes. Redirect added.

Note: As I think about it, all these OpenAI SDK use cases should be how-tos rather than configuration examples.

Preview Links

https://deploy-preview-4208--kongdeveloper.netlify.app//how-to/route-openai-sdk-to-multiple-providers

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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