docs: Add gtk.enable to home manager instructions

[dacid44]

When trying to set this up in NixOS, I noticed that you also have to enable GTK management in home manager for the GTK theming to work. This could probably also be solved by instead making the home manager module automatically set gtk.enable = true if the gtk segment is enabled.

[ozwaldorf]

I definitely agree, in fact I think the example nixos derivation should probably enable/install all apps that get configureed (at least, the one in the flake, as its built as a ci check and some errors are missed until the app is actually enabled in hm)

[ozwaldorf]

Its a good point though on noting enable must be set in documentation, versus using something like app.enable = lib.mkForce true.

It would probably be more convenient to force enable, but are there any drawbacks or blocked patterns here? I cant think of much reason a user would want to enable the themes and not the app. I'm slightly leaning towards mkforce but curious if you have any thoughts

[dacid44]

What does mkForce do exactly? I'm still pretty new to Nix and especially to NixOS/home-manager.