docs(configuration): add configuration for a perSystem flake-parts option

[shivaraj-bh]

Here’s what the debug.options and currentSystem.options looks like in services-flake’s dev flake:

services-flake/dev on  main [!] via ❄️  impure (nix-shell-env) took 2m9s
❯ nix repl
Welcome to Nix 2.18.2. Type :? for help.

nix-repl> :lf .
warning: Git tree '/Users/shivaraj/oss/services-flake' is dirty
Added 23 variables.

nix-repl> currentSystem.options
{ _module = { ... }; allModuleArgs = { ... }; apps = { ... }; checks = { ... }; debug = { ... }; devShells = { ... }; flake-root = { ... }; formatter = { ... }; legacyPackages = { ... }; packages = { ... }; pre-commit = { ... }; treefmt = { ... }; }

nix-repl> debug.options
{ _module = { ... }; allSystems = { ... }; debug = { ... }; flake = { ... }; perInput = { ... }; perSystem = { ... }; systems = { ... }; transposition = { ... }; }

currentSystem.options comprises of pre-commit and treefmt options, whereas debug.options doesn’t.