Fix /docs/guides/lemur redirect destination

[devin-ai-integration]

Summary

Updates the redirect destination for /docs/guides/lemur from the external partners page to the internal LLM Gateway overview page.

Before: /docs/guides/lemur → https://www.assemblyai.com/partners/anthropic
After: /docs/guides/lemur → /docs/llm-gateway/overview

Review & Testing Checklist for Human

• Verify /docs/llm-gateway/overview exists and is the correct destination for users looking for LeMUR-related content
• After deployment, test that visiting assemblyai.com/docs/guides/lemur correctly redirects to the LLM Gateway overview page

Notes

This is a follow-up fix to PR #544 where the redirect destination was incorrectly set.

Link to Devin run: https://app.devin.ai/sessions/20ba86d4b7f44aca8ff3fde5b67a2f7a
Requested by: Lee Vaughn (@LeeVaughn)

[devin-ai-integration]

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