feat: add SSE streaming API Reference example

[devin-ai-integration]

Summary

Adds a new sse/ example project demonstrating server-sent events (SSE) streaming endpoints in an API Reference. The OpenAPI spec uses the x-fern-streaming extension with three SSE patterns:

• Basic SSE — POST /plants/{plantId}/monitor streams real-time sensor readings
• SSE with terminator — POST /orders/{orderId}/track streams order status updates with a [DONE] terminator
• Stream condition — POST /plants/chat supports both streamed and non-streamed responses via a stream request parameter

The example also includes standard REST endpoints (CRUD for plants and orders) alongside the streaming endpoints, following the plant-themed content convention.

Follows the same structure as existing examples (grpc, graphql): fern.config.json, generators.yml, docs.yml, openapi.yaml, welcome page, API reference overview, shared assets, and styles.css.

CI workflows (check.yml, publish-docs.yml, preview-docs.yml) and README.md are updated to include the new sse project.

Review & Testing Checklist for Human

• Verify the live demo at sse.docs.buildwithfern.com renders correctly after merge (publishing happens on push to main)
• Confirm the streaming endpoints show SSE-specific UI in the API Reference tab
• Check that the stream condition endpoint (/plants/chat) shows both streamed and non-streamed response variants

Notes

• fern check passes locally with 0 errors (1 warning about contrast ratio, same as all other examples using the shared color palette)
• Uses the same fern.config.json version (5.35.0) and organization (fern-docs-examples) as the grpc and graphql examples

Link to Devin session: https://app.devin.ai/sessions/411780f34776464e8c69ab07efb9b6c1
Requested by: @devalog

[devin-ai-integration]

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[github-actions]

🌿 Preview sse: https://fern-docs-examples-preview-pr14-sse.docs.buildwithfern.com

Here are the markdown pages you've updated:

• api-reference
• welcome