feat(ai-gateway): Add codex how-to

[tomek-labuk]

Description

Fixes #issue

Preview Links

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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[jbaross]

LGTM

[jbaross]

LGTM from the content side.

For the config, I was able to follow this guide and get a response from openai at the 'Verify the AI Model' step but could not use codex itself. I got an error unexpected status 401 Unauthorized: Missing bearer or basic authentication in header