Add a “Global verbosity option” example to README / documentation

### Description

### Type of issue
Feature request / documentation sample

### Description
Many CLI tools follow a pattern where a global --verbosity (short -v) option controls log or output verbosity across all subcommands.
Currently, README and docs do not have a clear example showing how to implement this using System.CommandLine.

### Suggested improvement
Add a sample to README or samples folder demonstrating:

```csharp
var verbosity = new Option(new [] { "-v", "--verbosity" }, "Set verbosity");
root.AddGlobalOption(verbosity);
// ... handler that reads verbosity value or sets default if no value given

### Reproduction Steps

### Type of issue
Feature request / documentation sample

### Description
Many CLI tools follow a pattern where a global --verbosity (short -v) option controls log or output verbosity across all subcommands.
Currently, README and docs do not have a clear example showing how to implement this using System.CommandLine.

### Suggested improvement
Add a sample to README or samples folder demonstrating:

```csharp
var verbosity = new Option(new [] { "-v", "--verbosity" }, "Set verbosity");
root.AddGlobalOption(verbosity);
// ... handler that reads verbosity value or sets default if no value given

### Expected behavior

The SDK should locate global.json starting from:
The solution directory (if a .sln exists),
Else the project directory (if a .csproj exists),
Else the current working directory.
Then it should search parent folders upward and use the first global.json found to determine the SDK version.

### Actual behavior

The SDK currently starts searching for global.json from the current working directory only,
without first checking for a solution or project directory.
As a result, running dotnet build or dotnet restore from subfolders may ignore the intended global.json at the solution level.
This leads to unexpected SDK versions being used in multi-project setups.

### Regression?

The SDK currently starts searching for global.json from the current working directory only,
without first checking for a solution or project directory.
As a result, running dotnet build or dotnet restore from subfolders may ignore the intended global.json at the solution level.
This leads to unexpected SDK versions being used in multi-project setups.

### Known Workarounds

The SDK currently starts searching for global.json from the current working directory only,
without first checking for a solution or project directory.
As a result, running dotnet build or dotnet restore from subfolders may ignore the intended global.json at the solution level.
This leads to unexpected SDK versions being used in multi-project setups.

### Configuration

The SDK currently starts searching for global.json from the current working directory only,
without first checking for a solution or project directory.
As a result, running dotnet build or dotnet restore from subfolders may ignore the intended global.json at the solution level.
This leads to unexpected SDK versions being used in multi-project setups.

### Other information

The SDK currently starts searching for global.json from the current working directory only,
without first checking for a solution or project directory.
As a result, running dotnet build or dotnet restore from subfolders may ignore the intended global.json at the solution level.
This leads to unexpected SDK versions being used in multi-project setups.