Document scalable subdomains

[ethanpalm]

Documentation changes

This PR updates custom subpath pages to mention that you can use any custom subpath, not just /docs. It also adds the Vercel rewrites generator from https://github.com/mintlify/docs/pull/699/files to the Vercel page.

Closes https://linear.app/mintlify/issue/DOC-134/ungating-subdomains

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