chore(docs): refresh README — update messaging, remove Spec Generators, replace Slack

[matlegault]

Description

Refs FER-10749

Refreshes the fern-api/fern README to better align with the current site messaging and remove outdated references.

Changes Made

• Updated tagline to "Docs, SDKs, and CLIs for your API. Designed for the AI era." — consistent with buildwithfern.com
• Updated feature bullets to lead with agent-friendly docs, llms.txt, and MCP server; expanded SDK languages to include Swift and Rust
• Removed the "Spec Generators" section as requested
• Removed Slack community link (Slack was deleted) and replaced with X, GitHub Issues, and LinkedIn
• Removed "SDK Generators" subheading since Spec Generators section is gone and there's only one generator table now
• Updated supported specs — replaced OpenRPC with GraphQL to match the site

Note for repo maintainers: The GitHub repo "About" description is currently "Input OpenAPI. Output SDKs and Docs." — consider updating it to "Docs, SDKs, and CLIs for your API. Designed for the AI era." to match.

Testing

• Manual review of rendered markdown
• No code changes — documentation only

Link to Devin session: https://app.devin.ai/sessions/b6c745efa79b45d0af019a552d7bb049
Requested by: @matlegault

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