fix: update SDK quickstart OpenAPI directory structure to match actual CLI output

[devin-ai-integration]

Summary

Updates the OpenAPI tab in the SDK Quickstart page to show the correct directory structure produced by fern init --openapi. The previous docs showed a nested api/ folder with an openapi/ subfolder, but the actual CLI output is a flat structure:

fern/
├── .gitignore
├── fern.config.json
└── generators.yml

Only one file changed: fern/products/sdks/fern-folder.mdx.

Review & Testing Checklist for Human

• Verify the documented structure matches the current CLI: Run fern init --openapi path/to/spec.yml --organization test and confirm the output matches what the docs now show (flat structure with .gitignore, fern.config.json, generators.yml)
• Check whether .gitignore should also be added to the Fern Definition tab for consistency — the Fern Definition tab currently doesn't list it
• Check the preview deployment to verify the rendered file tree component looks correct

Notes

• The pre-existing Vale warning on line 93 (Avoid time-relative terms like 'Now') is unrelated to this change
• Link to Devin Session
• Requested by: Fern Support

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[fern-api]

Requested by: kapil
Slack thread: View conversation

[github-actions]

🌿 Preview your docs: https://fern-preview-db01137a-abd4-4dfe-acfa-5f8d7c901566.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/overview/quickstart