Document user files of nix

doc/manual/src/command-ref/files/user-profile-link.md

@@ -0,0 +1,16 @@
+# User profile link
+
+A symbolic link to the user's current profile. 
+
+- `~/.nix-profile`
+- `$XDG_STATE_HOME/nix/profile` if [`use-xdg-base-directories`] is set to `true`.
+
+By default, this symlink points to:
+
+- `$XDG_STATE_HOME/nix/profiles/default` for regular users
+- `$NIX_STATE_DIR/profiles/per-user/root/profile` for `root`
+
+The `PATH` environment variable should include `/bin` subdirectory of the profile link (e.g. `~/.nix-profile/bin`) for the user environment to be visible to the user.
+The [installer](@docroot@/installation/installing-binary.md) sets this up by default.
+
+[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories