Documentation changes

This PR explains steps needed to configure routing logic for using a custom subdomain with Webflow + Cloudflare.

Closes https://linear.app/mintlify/issue/DOC-40/custom-subdirectory-update

Dependent on https://github.com/mintlify/docs/pull/791/files Merge #791 first to avoid merge conflicts.

For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful