Add built-in example for defining a global --verbosity / -v option

[abhiramch018]

Type of issue

Feature request / documentation enhancement

Description

System.CommandLine already supports global options, but there’s no sample showing
how to define a global --verbosity (-v) option that applies to all subcommands
and defaults to Diagnostic when specified without a value.

Suggested improvement

Add a short example to the README or samples folder demonstrating:

var verbosity = new Option<string>(
    new[] { "-v", "--verbosity" },
    "Set output verbosity (quiet, minimal, normal, detailed, diagnostic)")
{
    Arity = ArgumentArity.ZeroOrOne
};
root.AddGlobalOption(verbosity);
Why this helps

--verbosity is a standard .NET CLI pattern, so showing how to implement it helps developers align their own tools with .NET conventions.