docs: Add Gemini Image Generation tool documentation

[usnavy13]

Summary

Adds documentation for the new Gemini Image Generation tool:

Changes

1. New tool configuration page (pages/docs/configuration/tools/gemini_image_gen.mdx)

   • Setup instructions for both Gemini API and Vertex AI
   • Configuration options (model selection, environment variables)
   • Feature overview (text-to-image, image context, editing)
   • Best practices for prompt writing and image editing
   • Usage examples and error handling

2. Updated image generation features page (pages/docs/features/image_gen.mdx)

   • Added Gemini Image Tools to the comparison table
   • Added new section (Section 2) with overview, parameters, and setup
   • Renumbered existing sections (DALL·E → 3, Stable Diffusion → 4, Flux → 5, MCP → 6)

Related

This documentation corresponds to the feature PR: https://github.com/danny-avila/LibreChat/pull/10676

Change Type

• [x] Documentation update

Checklist

• [x] Documentation follows the existing format and style
• [x] Added entry to _meta.ts for navigation
• [x] Updated features overview page
• [x] Content is accurate and comprehensive

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