docs: add Cloudflare Workers reverse proxy tutorial

[fern-support]

Summary

Adds a "Reverse proxy" section to the existing "Bring your custom domain" page with a step-by-step Cloudflare Workers tutorial. The tutorial covers:

• Updating docs.yml with a custom domain
• Creating a Cloudflare Worker that proxies requests to https://app.buildwithfern.com with the x-fern-host header
• Adding a custom domain to the Worker
• Verifying the setup

Also adds a cross-link from the page intro to the new reverse proxy section.

Review & Testing Checklist for Human

• Verify the Worker code snippet works end-to-end (create a test Worker, set x-fern-host to a real docs site, confirm pages load)
• Confirm the Cloudflare dashboard navigation instructions match the current Cloudflare UI (Workers & Pages > Create, Settings > Domains & Routes)
• Review the page preview to ensure the Steps render correctly within the page structure

Notes

The reverse proxy approach is an alternative to the DNS-based setup methods already documented. The Worker code uses redirect: "manual" to preserve redirect responses from Fern rather than following them server-side.

Link to Devin session: https://app.devin.ai/sessions/9c1ed5c09c9f4e8c9f0266c56e0c698f
Requested by: @fern-support

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1777483781-cloudflare-reverse-proxy-tutoria.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

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