Launchers: a neutral example app in docs and tests

The Nix-package launcher example in docs and option examples is now
Signal (pkgs.signal-desktop, desktop ID "signal", sgnl:// and
signalcaptcha:// handlers). The check uses a made-up org.example.App
from a fake package (fake store path), covering the same Exec, action,
MimeType, source and missing-entry cases.

Screenshots: placeholder Steam account IDs in the docs, option example
and check.

modules/screenshots.nix

@@ -25,7 +25,7 @@ in {
     steamUserId = lib.mkOption {
       type = lib.types.nullOr (lib.types.strMatching "[1-9][0-9]*");
       default = null;
-      example = "80511808";
+      example = "12345678";
       description = ''
         Steam account ID: the folder name in ~/.local/share/Steam/userdata.
         `null`: the account last logged in, found at runtime.