Add migration guidance for Newtonsoft.Json JsonObject Title property

[Copilot]

This PR adds comprehensive documentation for migrating from Newtonsoft.Json's JsonObject(Title = "title") attribute to System.Text.Json.

Problem

The JsonObjectAttribute.Title property in Newtonsoft.Json is used to set the title in JSON schema generation, but System.Text.Json has no direct equivalent. Users migrating from Newtonsoft.Json needed guidance on how to achieve the same functionality.

Solution

Added migration guidance that explains:

1. No direct equivalent: System.Text.Json doesn't have a direct equivalent to JsonObjectAttribute.Title
2. Modern solution: Starting in .NET 9, users can achieve the same result using JsonSchemaExporter with TransformSchemaNode
3. OpenAPI context: Acknowledged that schema title customization is often needed for OpenAPI scenarios

Changes Made

Migration Guide Updates

• Extended the existing JsonObjectAttribute section in migrate-from-newtonsoft.md to include Title property guidance
• Added code example showing how to use TransformSchemaNode to set schema title
• Referenced the schema exporter documentation for more comprehensive information

Schema Exporter Documentation

• Added a new example in extract-schema.md demonstrating how to set custom schema titles
• Created JsonObjectTitleExample.cs with working code that shows the TransformSchemaNode pattern

Example Code

The solution demonstrates how to transform schema nodes to add title metadata:

JsonSchemaExporterOptions exporterOptions = new()
{
    TransformSchemaNode = (context, schema) =>
    {
        // Only set title for the root type (not properties)
        if (context.PropertyInfo is null && context.TypeInfo.Type == typeof(Person))
        {
            if (schema is not JsonObject jObj)
            {
                // Handle Boolean schema case
                JsonValueKind valueKind = schema.GetValueKind();
                schema = jObj = new JsonObject();
                if (valueKind is JsonValueKind.False)
                {
                    jObj.Add("not", true);
                }
            }
            
            // Set the title in the schema
            jObj.Insert(0, "title", "PersonTitle");
        }
        
        return schema;
    }
};

This approach provides the same functionality as Newtonsoft.Json's JsonObject(Title = "PersonTitle") but uses System.Text.Json's modern schema generation capabilities.

Fixes #45862.

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