Expand documentation of perSystem vs. nixpkgs.overlays

[devurandom]

Is your feature request related to a problem? Please describe.

I want to use an updated package from a nixpkgs PR instead of the version currently in nixos-unstable. I previously (before I used blueprint) used overlays for that. But https://numtide.github.io/blueprint/main/getting-started/configuration/#nixpkgsconfig recommends against nixpkgs.overlays and for perSystem. However, it describes neither why nor how.

Describe the solution you'd like

It would help me if https://numtide.github.io/blueprint/main/getting-started/configuration/#nixpkgsconfig would explain:

• what the problem with nixpkgs.overlays is
• what the benefit of perSystem is
• give a concrete example how I can override a package in pkgs (i.e. coming from inputs.nixpkgs) with one coming from another input

Describe alternatives you've considered

Stick with nixpkgs.overlays because I know how it works, and ignore the recommendation from the blueprint documentation.

[bestlem]

The main author of blueprint @zimbatm wrote this article https://zimbatm.com/notes/1000-instances-of-nixpkgs/