doc/nixos: Explain better NixOS specific Systemd stuff
doc/nixos: Explain better NixOS specific Systemd stuff

Divide the "Service Management" chapter into two sections. The 1st (the
original) explaining General, not NixOS specific ways to interact with
Systemd. The 2nd section, explaining NixOS specific things worth
knowing.

Explain in the 2nd section a bit NixOS modules and services of Nixpkgs,
and mention `systemd.user.services` option. Give an example
demonstrating how to enable imperatively an upstream provided unit file
for a user. Explain why `systemctl --user enable` doesn't work for the
long term on NixOS.

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