docs: add Pinterest Rich Pins metadata example (#77025)
In 75831c08e83abd120be3968ec32ee0e2a2c4d33d I added support for the
Pinterest rich pin meta tag, this change adds it to the
[generateMetadata](https://nextjs.org/docs/app/api-reference/functions/generate-metadata)
documentation.
- [x] Ran `pnpm prettier-fix` before commiting
