Remove Gemini 3 Pro Preview from LLM Gateway docs

[devin-ai-integration]

Summary

Removes all references to Gemini 3 Pro Preview (gemini-3-pro-preview) from the LLM Gateway overview page. Three table rows were deleted — one from each of the three model tables (by quality, by latency, and by provider under Google Gemini).

No other files in the repo contained references to this model.

Review & Testing Checklist for Human

• Confirm that Gemini 3 Pro Preview should be fully removed (not renamed/replaced with a GA version)
• Verify the rendered markdown tables still look correct after the row removals (no broken formatting)

Notes

• The "20+ models" wording in the overview paragraph was not changed since it remains accurate after this removal.

Link to Devin session: https://app.devin.ai/sessions/ed2ba4be91be45e9b3bb7a65a7a803e1
Requested by: @LeeVaughn

[devin-ai-integration]

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