chore(docs): simplify and consolidate README

[devin-ai-integration]

Description

Refs #6265

Simplifies and consolidates the README to align with the latest messaging and branding from buildwithfern.com. Reduces the README from 257 lines to ~80 lines while maintaining all essential information.

Link to Devin run: https://app.devin.ai/sessions/ad7e5dff67a9499fadf11e2ce472509c
Requested by: Catherine Deskur (catherine@buildwithfern.com)

Changes Made

• Updated tagline to match website: "Instant Docs and SDKs for your API"
• Replaced inline quickstart code blocks with links to SDK Quickstart and Docs Quickstart
• Added Customer Showcase table featuring ElevenLabs, Webflow, and Cohere with links to their SDKs and docs
• Restored generator version badges in organized tables (SDK, Server, Spec generators)
• Updated badges and Slack link to use buildwithfern.com/slack redirect
• Removed verbose sections: detailed Fern Definition examples, API First section, Inspiration section, CLI Reference table, Model Generators section

Human Review Checklist

• Verify documentation quickstart links are correct and accessible
• Verify Customer Showcase companies (ElevenLabs, Webflow, Cohere) and their SDK/docs links are accurate
• Confirm generator version badge Docker image names are correct
• Confirm removed content is acceptable (Fern Definition YAML examples, CLI Reference table, Model Generators)

Testing

• Lint checks passed (pnpm run check)
• Manual review of rendered README

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