Fix get sub router base get sub router routes

[willybrauner]

• Fix getSubRouterBase & getSubRouterRoutes in case LangService.showDefaultLangInUrl is set to false.
• Update documentation
• Update tests

  const path = getPathByRouteName(router.routes, "FooPage");
  // (if last param is false, '/:lang' will be not added) -> "/base/:lang/foo"
  const subBase = getSubRouterBase(path, router.base, true);
  // get subRoutes
  const subRoutes = getSubRouterRoutes(path, router.routes);