docs: Add COHERE_API_URL environment variable documentation

[busla]

Summary

This PR adds comprehensive documentation for the new COHERE_API_URL environment variable that enables custom Cohere API endpoint configuration for web search reranking functionality.

This documentation update complements the main LibreChat implementation PR: danny-avila/LibreChat#9544

Documentation Changes

1. Environment Variables Documentation (dotenv.mdx)

• Added COHERE_API_URL to the Web Search environment variables table
• Included clear description and usage example for LiteLLM proxy integration
• Maintains consistency with existing environment variable documentation patterns

2. Web Search Feature Documentation (web_search.mdx)

• Added COHERE_API_URL to the Quick Start environment variables section
• Enhanced Cohere reranker section with custom endpoint configuration instructions
• Included multiple deployment scenario examples:
  • LiteLLM proxy: COHERE_API_URL=http://litellm:8000/v1
  • Custom endpoints: COHERE_API_URL=https://your-custom-endpoint.com/v1
  • Default behavior when not specified

3. YAML Configuration Documentation (web_search.mdx)

• Added cohereApiUrl configuration option to the web search YAML example
• Created dedicated section explaining the cohereApiUrl parameter
• Included comprehensive usage examples for different deployment scenarios
• Maintains consistency with other optional configuration parameters

Key Features Documented

✅ Environment Variable Support: COHERE_API_URL for custom Cohere endpoints
✅ LiteLLM Integration: Specific examples for LiteLLM proxy deployments
✅ Enterprise Deployments: Custom endpoint configuration for enterprise setups
✅ Backwards Compatibility: Clear explanation that it's optional with sensible defaults
✅ Multiple Deployment Scenarios: Docker, Kubernetes, and custom proxy examples

Usage Examples Added

# LiteLLM proxy deployment
COHERE_API_URL=http://litellm:8000/v1

# Docker compose setup  
COHERE_API_URL=http://cohere-proxy:8000/v1

# Custom enterprise endpoint
COHERE_API_URL=https://your-cohere-endpoint.company.com/v1

Benefits

• Complete Coverage: Documentation covers all three main docs sections where Cohere is mentioned
• User-Friendly: Clear step-by-step instructions for different deployment scenarios
• Enterprise Ready: Addresses custom proxy and enterprise deployment needs
• Consistency: Follows established LibreChat documentation patterns and conventions

Files Modified

• pages/docs/configuration/dotenv.mdx - Environment variables reference
• pages/docs/features/web_search.mdx - Feature documentation and setup guide
• pages/docs/configuration/librechat_yaml/object_structure/web_search.mdx - YAML configuration reference

This documentation ensures users have comprehensive guidance for implementing custom Cohere endpoints in their LibreChat deployments, particularly for LiteLLM proxy integrations and enterprise environments.

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