chore(flake): move user inputs to separate dir + minor docs changes

flake/inputs/home/default.nix

@@ -0,0 +1,28 @@
+# User inputs
+
+/**
+  Inputs providing modules or packages for user configurations. Check next chapters to learn more.
+*/
+{
+  /**
+    System for managing a user environment.
+    Allows declarative configuration of user specific (non-global) packages and dotfiles.
+  */
+  home-manager = {
+    url = "github:nix-community/home-manager";
+  };
+  /**
+    Theming framework for NixOS
+  */
+  stylix = {
+    url = "github:nix-community/stylix";
+  };
+
+  /**
+    Flake for configuring Niri.
+    Used because home-manager does not support Niri yet.
+  */
+  niri = {
+    url = "github:sodiboo/niri-flake";
+  };
+}