Remove Cohere as an available model from Ask Fern docs

[paarthfern]

Summary

Removes all references to Cohere as an available AI model for Ask Fern across the public documentation:

• fern/snippets/ask-fern-config.mdx: Removed command-a (Cohere Command A) from the ai-search.model options list
• stainless-comparison.mdx: Removed "Cohere" from the AI chatbot feature row and the technology stack comparison table

References to Cohere as a customer (SDK examples, PDF export demos) are intentionally unchanged.

Review & Testing Checklist for Human

• Verify there are no other pages referencing Cohere as a model option for Ask Fern (a CLI changelog entry from 2025-03-27 mentioning Cohere Command R+ was intentionally kept as historical record)
• Confirm the preview deployment renders the config snippet and comparison tables correctly after the removal

Link to Devin session: https://app.devin.ai/sessions/96db9e98bc104770abe1d8aa45f9ceb4
Requested by: @paarthfern

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[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1774966919-remove-cohere-model-refs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/resources/stainless-comparison

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.