[Docs] Add PPR option to `next.config.js` docs (#58161)
### Description
Adds config for experimental PPR support
### Related Issue(s)/Incident
Prior art: #57444
### Best Way to Test
Visit → docs/app/api-reference/next-config-js/partial-prerendering
Co-authored-by: Amy Burns <5012825+timeyoutakeit@users.noreply.github.com>
Co-authored-by: Lee Robinson <9113740+leerob@users.noreply.github.com>
