treewide: add developer shell

Link: https://github.com/danth/stylix/pull/519

docs/src/SUMMARY.md

@@ -14,6 +14,7 @@
 # Contributing
 
 - [Commit Convention](commit_convention.md)
+- [Development Environment](development_environment.md)
 - [Adding modules](modules.md)
 - [Testbeds](testbeds.md)
 - [Style guide](styling.md)

docs/src/development_environment.md

@@ -0,0 +1,7 @@
+# Development Environment
+
+To enter the developer shell, run:
+
+```console
+nix develop
+```

flake.nix

@@ -86,6 +86,10 @@
         inherit (nixpkgs) lib;
         pkgs = nixpkgs.legacyPackages.${system};
       in {
+        devShells.default = pkgs.mkShell {
+          packages = [ inputs.home-manager.packages.${system}.default ];
+        };
+
         packages = let
             universalPackages = {
               docs = import ./docs { inherit pkgs inputs lib; };