Use better diagrams for kong gateway

[lena-larionova]

Description

Replacing the diagram on https://developer.konghq.com/gateway/ with dark mode/light mode diagrams in the same style as AI Gateway.

Preview Links

https://deploy-preview-1670--kongdeveloper.netlify.app/gateway/
Toggle between light and dark mode.

Checklist

• Every page is page one.
• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Updated sources.yaml. For more info, review track docs changes
• 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]

✅ Deploy Preview for kongdeveloper ready!

Name	Link
🔨 Latest commit	7e31c3b
🔍 Latest deploy log	https://app.netlify.com/projects/kongdeveloper/deploys/683a2a1683b95a0008feb1b5
😎 Deploy Preview	https://deploy-preview-1670--kongdeveloper.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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