doc: separate page for each target

[danth]

A separate page for each target would make it easier to browse through the documentation.

These pages could show more information about the target, such as:

• A longer description
• Which theming items it supports (wallpaper, colors, fonts, opacity, etc)
• Which platforms it supports (Home Manager / NixOS / Darwin)
• Options for each platform
• Whether manual setup is required for the module to work (see the Firefox module for example)

I have a few ideas for how this documentation could be generated:

• A hidden list which each target adds an item to:

  {
  stylix.documentationItems = [{
    targetName = "GNOME";
    targetDescription = ''
      This themes the status bar at the top of the screen, the app launcher,
       and the GDM login screen. For theming GNOME applications, see the GTK target.
    '';
    supportedItems = [ "fonts" "colors" ];
  }];
  }

  {
  stylix.documentationItems = [{
    targetName = "Firefox";
    supportedItems = [ "fonts" ];
    extraDocumentation = ''
      For this target to work, you need to fill in your profile name(s) using the option provided.
    '';
  }];
  }

• A hidden option called targets.«name».metadata, with the same format as above

• A separate Nix or JSON file, with the same format as above

[danth]

New suggestion: the extra documentation for each target could be a README.md in that folder, so it also shows up when browsing the source code