I suppose that actually moving rather than just forking out would need buy-in from Nixpkgs maintainers more generally, which, to me, implies running an RFC.
I see two seperate topics here.
Discoverability via „nicely rendered“ documentation of builtins. Maintainers of lib or similiar will be very thankfull i suppose.
Then the other topic, should we move .lib into a seperate repository. This sounds like an RFC tbh.
As roberth posted there is already something usable