feat(ai-gateway): Add how-tos for Gemini/Vertex SDKs

[tomek-labuk]

Description

Fixes #3788

Preview Links

https://deploy-preview-3786--kongdeveloper.netlify.app/how-to/use-gemini-sdk-chat/
https://deploy-preview-3786--kongdeveloper.netlify.app/how-to/use-vertex-sdk-chat/

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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[Guaris]

@tomek-labuk
I got
Error: building state: retrieve intermediate services (gemini-service): entity not found When trying to do the gemini prereq