docs: links for OpenAPI spec for both human-readable and JSON (#10216)
### Description
I realized I remove the link to the web-hosted JSON in
https://github.com/vercel/turborepo/pull/10172/files#diff-24fd2507ec67403cc099bc4ebb6b0b8ffda016451400a1437548956bb33be770.
This brings it back, with some slight reformatting now that we have a
human-readable viewer, too.
### Testing Instructions
👀