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

### 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:

```csharp
var verbosity = new Option(
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.