Add note about need to disable ppd on NixOS (#1240)

* Add Nixos power note to system76-software.md

ppd (enabled by default on Gnome NixOS installs) must be disabled for system76-power to start. Include that info in docs.

* Update system76-software.md

---------

Co-authored-by: Aaron Honeycutt <aaronhoneycutt@protonmail.com>

content/system76-software.md

@@ -270,3 +270,13 @@ hardware.system76.enableAll = true;
 ```bash
 sudo nixos-rebuild switch
 ```
+
+If your system has power-profiles-daemon installed (done by default on GNOME), you'll need to disable it for system76-power to start. Add this line to your `/etc/nixos/configuration.nix` file then rebuild the OS:
+
+```bash
+services.power-profiles-daemon.enable = false;
+```
+
+```bash
+sudo nixos-rebuild switch
+```