refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/configuration/ad-hoc-packages.section.md

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+# Ad-Hoc Package Management {#sec-ad-hoc-packages}
+
+With the command `nix-env`, you can install and uninstall packages from
+the command line. For instance, to install Mozilla Thunderbird:
+
+```ShellSession
+$ nix-env -iA nixos.thunderbird
+```
+
+If you invoke this as root, the package is installed in the Nix profile
+`/nix/var/nix/profiles/default` and visible to all users of the system;
+otherwise, the package ends up in
+`/nix/var/nix/profiles/per-user/username/profile` and is not visible to
+other users. The `-A` flag specifies the package by its attribute name;
+without it, the package is installed by matching against its package
+name (e.g. `thunderbird`). The latter is slower because it requires
+matching against all available Nix packages, and is ambiguous if there
+are multiple matching packages.
+
+Packages come from the NixOS channel. You typically upgrade a package by
+updating to the latest version of the NixOS channel:
+
+```ShellSession
+$ nix-channel --update nixos
+```
+
+and then running `nix-env -i` again. Other packages in the profile are
+*not* affected; this is the crucial difference with the declarative
+style of package management, where running `nixos-rebuild switch` causes
+all packages to be updated to their current versions in the NixOS
+channel. You can however upgrade all packages for which there is a newer
+version by doing:
+
+```ShellSession
+$ nix-env -u '*'
+```
+
+A package can be uninstalled using the `-e` flag:
+
+```ShellSession
+$ nix-env -e thunderbird
+```
+
+Finally, you can roll back an undesirable `nix-env` action:
+
+```ShellSession
+$ nix-env --rollback
+```
+
+`nix-env` has many more flags. For details, see the nix-env(1) manpage or
+the Nix manual.