chore: Move Bedrock and Gemini into LLM providers nav sections

[lena-larionova]

Description

Combined the "LLM Providers Integration Guides" and "Machine learning platform integration guides" sections into one. They're all technically LLM providers, and are listed that way on the landing pages for the AI Proxy and AI Proxy Advanced plugins already (see provider table): http://docs.konghq.com/hub/kong-inc/ai-proxy/

Combined the sections and added some extra cross links.

Request from Slack: https://kongstrong.slack.com/archives/C068E0KB16F/p1730324299268159?thread_ts=1730324265.528739&cid=C068E0KB16F

Testing instructions

Preview link:
https://deploy-preview-8103--kongdocs.netlify.app/hub/kong-inc/ai-proxy/
https://deploy-preview-8103--kongdocs.netlify.app/hub/kong-inc/ai-proxy-advanced/
https://deploy-preview-8103--kongdocs.netlify.app/gateway/latest/get-started/ai-gateway/

Checklist

• Review label added
• Conditional version tags added, if applicable.

[netlify]

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