docs: document cohereApiUrl/COHERE_API_URL for web search reranker

[busla]

Summary

Documents the new cohereApiUrl config field / COHERE_API_URL env var for the web search reranker, mirroring the existing jinaApiUrl/JINA_API_URL documentation:

• content/docs/configuration/librechat_yaml/object_structure/web_search.mdx — YAML example + property reference table
• content/docs/features/web_search.mdx — env var setup + YAML example
• content/docs/configuration/dotenv.mdx — env var reference table

This documents the feature added in danny-avila/LibreChat#15518 and danny-avila/agents#498, which lets the Cohere reranker point at a Cohere-compatible endpoint (e.g. a self-hosted LiteLLM proxy) instead of api.cohere.com.

English-only change, per this repo's localization convention (other languages are handled by translation automation).

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[busla]

The failing `analyze` check isn't related to this PR's content — the bundle analysis itself succeeded ("This PR introduced no changes to the JavaScript bundle!"), but the follow-up step that posts that result as a comment (peter-evans/create-or-update-comment) got a 403 Resource not accessible by integration from the GitHub API.

That's the standard fork-PR limitation: workflows triggered by pull_request from a fork run with a read-only GITHUB_TOKEN (no issues/pull-requests: write), so the bot can't post a comment regardless of what changed. This would fail the same way for any external PR, content-only or not — switching the workflow to pull_request_target (or granting it permissions: pull-requests: write) would fix it repo-wide.

Flagging so it doesn't block review of the docs change itself.

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