Before enabling dconf in Home Manager, dconf must be enabled in system
config.
Otherwise it will fail like this:
```
$ home-manager switch
Starting Home Manager activation
Activating checkFilesChanged
Activating checkLinkTargets
Activating writeBoundary
Activating installPackages
replacing old 'home-manager-path'
installing 'home-manager-path'
Activating dconfSettings
error: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit dconf.service not found.
```
Specifically, instead of
services.dbus.packages = with pkgs; [ gnome.dconf ];
we now recommend
programs.dconf.enable = true;
which does the same and more.
The docs implied that fish was not really supported, but if fish is
managed by Home Manager, the generated config does use fenv to source
the session-vars file. Update the installation instructions and FAQ to
mention that fish does work, and mention fenv in the README.
