refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/development/testing-installer.chapter.md

@@ -0,0 +1,18 @@
+# Testing the Installer {#ch-testing-installer}
+
+Building, burning, and booting from an installation CD is rather
+tedious, so here is a quick way to see if the installer works properly:
+
+```ShellSession
+# mount -t tmpfs none /mnt
+# nixos-generate-config --root /mnt
+$ nix-build '<nixpkgs>' -A nixos-install
+# ./result/bin/nixos-install
+```
+
+To start a login shell in the new NixOS installation in `/mnt`:
+
+```ShellSession
+$ nix-build '<nixpkgs>' -A nixos-enter
+# ./result/bin/nixos-enter
+```