Add note about nginx Brotli compatibility for subpath reverse proxy setups

[alecharmon]

Summary

Adds a warning note to the Subpath section of the custom domain setup page, documenting a known issue where nginx reverse proxies can cause HTTP/2 partial transfer errors when proxying Fern docs.

Root cause: Fern's CDN (Cloudflare/Vercel) serves Brotli-compressed responses when the client advertises br support. nginx lacks native Brotli support, so when it proxies these responses it can corrupt the stream over HTTP/2. The fix is to configure nginx to only request encodings it supports natively (gzip,deflate).

This was reported by a customer (SignalWire) who experienced the issue in production.

Review & Testing Checklist for Human

• Verify the warning renders correctly on the preview — it should appear inside the Subpath accordion, after the setup steps
• Confirm the technical explanation is accurate and the nginx directive proxy_set_header Accept-Encoding "gzip,deflate"; is the right recommendation

Notes

• The warning is scoped to the Subpath section since that's the most common reverse proxy use case, but the issue could technically affect any nginx reverse proxy setup pointing at Fern docs
• Vale style checks pass cleanly
• Minor trailing whitespace cleanup on the last line of the file

Link to Devin session: https://app.devin.ai/sessions/5d76edcab8864bdcb82362654e203ef1
Requested by: @alecharmon

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[github-actions]

🌿 Preview your docs: https://fern-preview-04f3bc43-fe88-4f38-bf03-5d0766c1ed39.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/preview-publish/setting-up-your-domain