[Feature] More concise PnP specification

[pauldraper]

Is this the authoritative PnP spec? https://yarnpkg.com/advanced/pnpapi

If so, a a couple suggestions:

1. More clearly delineate the PnP specification vs Yarn extensions. Namely, separate them in different documents entirely.

2. Consider a separate or more prominent location for it. Right now, the PnP spec is buried in "Yarn Advanced."

I think this changes would help improve the adoption of PnP as the standard Node.js module resolution API.

[merceyz]

Fixed in #4671