Add LLM Gateway FAQ section with LeMUR migration article

[devin-ai-integration]

Summary

Adds a new "LLM Gateway" section to the FAQ with an article titled "How do I switch from using LeMUR to using LLM Gateway?" The article briefly explains the three key changes needed (include transcript text directly, update model names, modify response handling) and links to the detailed migration guide for a full walkthrough.

Review & Testing Checklist for Human

• Verify the link /docs/llm-gateway/migration-from-lemur resolves correctly in the deployed preview
• Confirm the FAQ content accurately summarizes the migration process
• Check that the "LLM Gateway" section appears in the expected position in the FAQ sidebar (after "Speech understanding", before "Playground")

Notes

Requested by: @LeeVaughn
Devin session: https://app.devin.ai/sessions/799413b13e97412facc8b3cc9bfdaab2

[devin-ai-integration]

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