docs: document configurable Cohere reranker API URL

[npeham]

Summary

Documentation for danny-avila/LibreChat#12969

This PR adds support for configuring a custom Cohere API URL in the web search configuration, enabling users to use Cohere-compatible endpoints such as Azure AI Foundry's serverless deployments.

Key Changes

• Added cohereApiUrl configuration option to the web search settings in librechat_yaml
• Updated documentation to include the new COHERE_API_URL environment variable
• Added clarification that the Cohere API URL is optional and defaults to https://api.cohere.com/v2/rerank
• Updated configuration examples to show the new cohereApiUrl parameter
• Added notes about Azure AI Foundry serverless Cohere endpoint support
• Updated setup instructions to include the optional Cohere API URL configuration step

Implementation Details

• The cohereApiUrl parameter follows the same pattern as the existing jinaApiUrl configuration
• It accepts an environment variable reference (e.g., ${COHERE_API_URL})
• Users can optionally provide the URL via the UI if not set in environment variables
• The feature is fully backward compatible - existing configurations without this parameter will continue to work with the default Cohere endpoint

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