Vercel proxy set up docs

[ethanpalm]

Documentation changes

This PR updates the subdomain docs with information about proxying on top of Vercel deployments. Also makes some general improvements to the Vercel page.

Closes https://linear.app/mintlify/issue/DOC-1/update-the-vercel-custom-subdirectory-documentation
Closes https://linear.app/mintlify/issue/DOC-86/proper-proxy-set-up-to-work-with-vercel

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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

[ricardonunez-io]

LGTM 💯 Appreciate you writing this out!