docs: add example OpenAPI spec to SDK quickstart pages

[devin-ai-integration]

Summary

Adds an "Example (Petstore)" tab to the existing <CodeBlocks> in the SDK quickstart init step. Users who don't have their own OpenAPI spec can use the public Swagger Petstore URL to try out SDK generation immediately.

Previously the quickstarts had a Fern Definition option with a built-in imdb.yml example, but it was removed when the init flow was streamlined to only show the OpenAPI path. This restores a "try it out" path as a third code block alongside the existing local and URL options.

The change is in the shared init-fern-folder.mdx snippet, so it appears on all language-specific quickstart pages and the top-level SDKs quickstart.

Review & Testing Checklist for Human

• Verify the third code block tab renders correctly on a quickstart page (e.g., /learn/sdks/generators/typescript/quickstart)
• Run fern init --openapi https://petstore3.swagger.io/api/v3/openapi.json --organization my-org and confirm fern generate produces a valid SDK

Notes

Uses the Swagger Petstore v3 spec — a well-known, stable public spec maintained by the Swagger/OpenAPI team.

Link to Devin session: https://app.devin.ai/sessions/d2f0eef7747d47958b1f81c0c2163fd6
Requested by: @devalog

[devin-ai-integration]

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