nixos/doc: document defining basic systemd services (#418385)

nixos/doc/manual/administration/service-mgmt.chapter.md

@@ -121,7 +121,28 @@ Hence [garbage collection](#sec-nix-gc) will remove that file and you
 will wind up with a broken symlink in your systemd configuration, which
 in turn will not make the service / timer start on login.
 
-## Template units {#sect-nixos-systemd-template-units}
+### Defining custom services {#sect-nixos-systemd-custom-services}
+
+You can define services by adding them to `systemd.services`:
+
+```nix
+systemd.services.myservice = {
+  after = [ "network-online.target" ];
+  requires = [ "network-online.target" ];
+
+  before = [ "multi-user.target" ];
+  wantedBy = [ "multi-user.target" ];
+
+  serviceConfig = {
+    ExecStart = "...";
+  };
+};
+```
+
+If you want to specify a multi-line script for `ExecStart`,
+you may want to use `pkgs.writeShellScript`.
+
+### Template units {#sect-nixos-systemd-template-units}
 
 systemd supports templated units where a base unit can be started multiple
 times with a different parameter. The syntax to accomplish this is

nixos/doc/manual/redirects.json

@@ -1556,6 +1556,9 @@
   "sect-nixos-systemd-nixos": [
     "index.html#sect-nixos-systemd-nixos"
   ],
+  "sect-nixos-systemd-custom-services": [
+    "index.html#sect-nixos-systemd-custom-services"
+  ],
   "sect-nixos-systemd-template-units": [
     "index.html#sect-nixos-systemd-template-units"
   ],